diff --git a/test/test_collector.py b/test/test_collector.py
index 1be9bc9ed15..548965909ea 100644
--- a/test/test_collector.py
+++ b/test/test_collector.py
@@ -1712,6 +1712,186 @@ def env_fn():
                 total_frames=frames_per_batch * 100,
             )
 
+    class FixedIDEnv(EnvBase):
+        """
+        A simple mock environment that returns a fixed ID as its sole observation.
+
+        This environment is designed to test MultiSyncDataCollector ordering.
+        Each environment instance is initialized with a unique env_id, which it
+        returns as the observation at every step.
+        """
+
+        def __init__(
+            self,
+            env_id: int,
+            max_steps: int = 10,
+            sleep_odd_only: bool = False,
+            **kwargs,
+        ):
+            """
+            Args:
+                env_id: The ID to return as observation. This will be returned as a tensor.
+                max_steps: Maximum number of steps before the environment terminates.
+            """
+            super().__init__(device="cpu", batch_size=torch.Size([]))
+            self.env_id = env_id
+            self.max_steps = max_steps
+            self.sleep_odd_only = sleep_odd_only
+            self._step_count = 0
+
+            # Define specs
+            self.observation_spec = Composite(
+                observation=Unbounded(shape=(1,), dtype=torch.float32)
+            )
+            self.action_spec = Composite(
+                action=Unbounded(shape=(1,), dtype=torch.float32)
+            )
+            self.reward_spec = Composite(
+                reward=Unbounded(shape=(1,), dtype=torch.float32)
+            )
+            self.done_spec = Composite(
+                done=Unbounded(shape=(1,), dtype=torch.bool),
+                terminated=Unbounded(shape=(1,), dtype=torch.bool),
+                truncated=Unbounded(shape=(1,), dtype=torch.bool),
+            )
+
+        def _reset(self, tensordict: TensorDict | None = None, **kwargs) -> TensorDict:
+            """Reset the environment and return initial observation."""
+            # Add sleep to simulate real-world timing variations
+            # This helps test that the collector properly handles different reset times
+            if not self.sleep_odd_only:
+                # Random sleep up to 10ms
+                time.sleep(torch.rand(1).item() * 0.01)
+            elif self.env_id % 2 == 1:
+                time.sleep(1)
+
+            self._step_count = 0
+            return TensorDict(
+                {
+                    "observation": torch.tensor(
+                        [float(self.env_id)], dtype=torch.float32
+                    ),
+                    "done": torch.tensor([False], dtype=torch.bool),
+                    "terminated": torch.tensor([False], dtype=torch.bool),
+                    "truncated": torch.tensor([False], dtype=torch.bool),
+                },
+                batch_size=self.batch_size,
+            )
+
+        def _step(self, tensordict: TensorDict) -> TensorDict:
+            """Execute one step and return the env_id as observation."""
+            self._step_count += 1
+            done = self._step_count >= self.max_steps
+
+            if self.sleep_odd_only and self.env_id % 2 == 1:
+                time.sleep(1)
+
+            return TensorDict(
+                {
+                    "observation": torch.tensor(
+                        [float(self.env_id)], dtype=torch.float32
+                    ),
+                    "reward": torch.tensor([1.0], dtype=torch.float32),
+                    "done": torch.tensor([done], dtype=torch.bool),
+                    "terminated": torch.tensor([done], dtype=torch.bool),
+                    "truncated": torch.tensor([False], dtype=torch.bool),
+                },
+                batch_size=self.batch_size,
+            )
+
+        def _set_seed(self, seed: int | None) -> int | None:
+            """Set the seed for reproducibility."""
+            if seed is not None:
+                torch.manual_seed(seed)
+            return seed
+
+    @pytest.mark.parametrize("num_envs,n_steps", [(8, 5)])
+    @pytest.mark.parametrize("with_preempt", [False, True])
+    @pytest.mark.parametrize("cat_results", ["stack", -1])
+    def test_multi_sync_data_collector_ordering(
+        self, num_envs: int, n_steps: int, with_preempt: bool, cat_results: str | int
+    ):
+        """
+        Test that MultiSyncDataCollector returns data in the correct order.
+
+        We create num_envs environments, each returning its env_id as the observation.
+        After collection, we verify that the observations correspond to the correct env_ids in order
+        """
+        if with_preempt and IS_OSX:
+            pytest.skip(
+                "Cannot use preemption on OSX due to Queue.qsize() not being implemented on this platform."
+            )
+
+        # Create environment factories using partial - one for each env_id
+        # This pattern mirrors CrossPlayEvaluator._rollout usage
+        env_factories = [
+            functools.partial(
+                self.FixedIDEnv, env_id=i, max_steps=10, sleep_odd_only=with_preempt
+            )
+            for i in range(num_envs)
+        ]
+
+        # Initialize MultiSyncDataCollector
+        collector = MultiSyncDataCollector(
+            create_env_fn=env_factories,
+            frames_per_batch=num_envs * n_steps,
+            total_frames=num_envs * n_steps,
+            device="cpu",
+            preemptive_threshold=0.5 if with_preempt else None,
+            cat_results=cat_results,
+            init_random_frames=n_steps,  # no need of a policy
+            use_buffers=True,
+        )
+
+        # Collect one batch
+        for batch in collector:
+            # Verify that each environment's observations match its env_id
+            # batch has shape [num_envs, frames_per_env]
+            # In the pre-emption case, we have that envs with odd ids are order of magnitude slower.
+            # These should be skipped by pre-emption (since they are the 50% slowest)
+
+            # Recover rectangular shape of batch to uniform checks
+            if cat_results != "stack":
+                if not with_preempt:
+                    batch = batch.reshape(num_envs, n_steps)
+                else:
+                    traj_ids = batch["collector", "traj_ids"]
+                    traj_ids[traj_ids == 0] = 99  # avoid using traj_ids = 0
+                    # Split trajectories to recover correct shape
+                    # thanks to having a single trajectory per env
+                    # Pads with zeros!
+                    batch = split_trajectories(
+                        batch, trajectory_key=("collector", "traj_ids")
+                    )
+                    # Use -1 for padding to uniform with other preemption
+                    is_padded = batch["collector", "traj_ids"] == 0
+                    batch[is_padded] = -1
+
+            #
+            for env_idx in range(num_envs):
+                if with_preempt and env_idx % 2 == 1:
+                    # This is a slow env, should have been preempted after first step
+                    assert (batch["collector", "traj_ids"][env_idx, 1:] == -1).all()
+                    continue
+                # This is a fast env, no preemption happened
+                assert (batch["collector", "traj_ids"][env_idx] != -1).all()
+
+                env_data = batch[env_idx]
+                observations = env_data["observation"]
+                # All observations from this environment should equal its env_id
+                expected_id = float(env_idx)
+                actual_ids = observations.flatten().unique()
+
+                assert len(actual_ids) == 1, (
+                    f"Env {env_idx} should only produce observations with value {expected_id}, "
+                    f"but got {actual_ids.tolist()}"
+                )
+                assert (
+                    actual_ids[0].item() == expected_id
+                ), f"Environment {env_idx} should produce observation {expected_id}, but got {actual_ids[0].item()}"
+
+        collector.shutdown()
+
 
 class TestCollectorDevices:
     class DeviceLessEnv(EnvBase):
diff --git a/torchrl/collectors/_multi_sync.py b/torchrl/collectors/_multi_sync.py
index 1f756a8b26d..c423f1f02c6 100644
--- a/torchrl/collectors/_multi_sync.py
+++ b/torchrl/collectors/_multi_sync.py
@@ -216,7 +216,7 @@ def iterator(self) -> Iterator[TensorDictBase]:
         if cat_results is None:
             cat_results = "stack"
 
-        self.buffers = {}
+        self.buffers = [None for _ in range(self.num_workers)]
         dones = [False for _ in range(self.num_workers)]
         workers_frames = [0 for _ in range(self.num_workers)]
         same_device = None
@@ -236,7 +236,6 @@ def iterator(self) -> Iterator[TensorDictBase]:
                     msg = "continue_random"
                 else:
                     msg = "continue"
-                # Debug: sending 'continue'
                 self.pipes[idx].send((None, msg))
 
             self._iter += 1
@@ -299,8 +298,11 @@ def iterator(self) -> Iterator[TensorDictBase]:
                 if preempt:
                     # mask buffers if cat, and create a mask if stack
                     if cat_results != "stack":
-                        buffers = {}
-                        for worker_idx, buffer in self.buffers.items():
+                        buffers = [None] * self.num_workers
+                        for worker_idx, buffer in enumerate(self.buffers):
+                            # Skip pre-empted envs:
+                            if buffer is None:
+                                continue
                             valid = buffer.get(("collector", "traj_ids")) != -1
                             if valid.ndim > 2:
                                 valid = valid.flatten(0, -2)
@@ -308,7 +310,7 @@ def iterator(self) -> Iterator[TensorDictBase]:
                                 valid = valid.any(0)
                             buffers[worker_idx] = buffer[..., valid]
                     else:
-                        for buffer in self.buffers.values():
+                        for buffer in filter(lambda x: x is not None, self.buffers):
                             with buffer.unlock_():
                                 buffer.set(
                                     ("collector", "mask"),
@@ -320,7 +322,7 @@ def iterator(self) -> Iterator[TensorDictBase]:
 
                 # Skip frame counting if this worker didn't send data this iteration
                 # (happens when reusing buffers or on first iteration with some workers)
-                if idx not in buffers:
+                if self.buffers[idx] is None:
                     continue
 
                 workers_frames[idx] = workers_frames[idx] + buffers[idx].numel()
@@ -331,18 +333,18 @@ def iterator(self) -> Iterator[TensorDictBase]:
             if self.replay_buffer is not None:
                 yield
                 self._frames += sum(
-                    [
-                        self.frames_per_batch_worker(worker_idx=worker_idx)
-                        for worker_idx in range(self.num_workers)
-                    ]
+                    self.frames_per_batch_worker(worker_idx)
+                    for worker_idx in range(self.num_workers)
                 )
                 continue
 
             # we have to correct the traj_ids to make sure that they don't overlap
             # We can count the number of frames collected for free in this loop
             n_collected = 0
-            for idx in buffers.keys():
+            for idx in range(self.num_workers):
                 buffer = buffers[idx]
+                if buffer is None:
+                    continue
                 traj_ids = buffer.get(("collector", "traj_ids"))
                 if preempt:
                     if cat_results == "stack":
@@ -356,7 +358,7 @@ def iterator(self) -> Iterator[TensorDictBase]:
             if same_device is None:
                 prev_device = None
                 same_device = True
-                for item in self.buffers.values():
+                for item in filter(lambda x: x is not None, self.buffers):
                     if prev_device is None:
                         prev_device = item.device
                     else:
@@ -367,10 +369,12 @@ def iterator(self) -> Iterator[TensorDictBase]:
                     torch.stack if self._use_buffers else TensorDict.maybe_dense_stack
                 )
                 if same_device:
-                    self.out_buffer = stack(list(buffers.values()), 0)
+                    self.out_buffer = stack(
+                        [item for item in buffers if item is not None], 0
+                    )
                 else:
                     self.out_buffer = stack(
-                        [item.cpu() for item in buffers.values()], 0
+                        [item.cpu() for item in buffers if item is not None], 0
                     )
             else:
                 if self._use_buffers is None:
@@ -383,10 +387,13 @@ def iterator(self) -> Iterator[TensorDictBase]:
                     )
                 try:
                     if same_device:
-                        self.out_buffer = torch.cat(list(buffers.values()), cat_results)
+                        self.out_buffer = torch.cat(
+                            [item for item in buffers if item is not None], cat_results
+                        )
                     else:
                         self.out_buffer = torch.cat(
-                            [item.cpu() for item in buffers.values()], cat_results
+                            [item.cpu() for item in buffers if item is not None],
+                            cat_results,
                         )
                 except RuntimeError as err:
                     if (