diff --git a/src/httpx2/httpx2/_models.py b/src/httpx2/httpx2/_models.py
index e6aeabd6..b50adbd9 100644
--- a/src/httpx2/httpx2/_models.py
+++ b/src/httpx2/httpx2/_models.py
@@ -45,7 +45,7 @@
     ResponseExtensions,
     SyncByteStream,
 )
-from ._urls import URL
+from ._urls import URL, QueryParams
 from ._utils import to_bytes_or_str, to_str
 
 __all__ = ["Cookies", "Headers", "Request", "Response"]
@@ -385,7 +385,18 @@ def __init__(
         extensions: RequestExtensions | None = None,
     ) -> None:
         self.method = method.upper()
-        self.url = URL(url) if params is None else URL(url, params=params)
+        if params is None:
+            self.url = URL(url)
+        else:
+            base_url = URL(url)
+            params_obj = QueryParams(params)
+            if params_obj:
+                new_params_bytes = str(params_obj).encode("ascii")
+                existing_query = base_url.query  # b"" if no query
+                merged_query = (existing_query + b"&" + new_params_bytes) if existing_query else new_params_bytes
+                self.url = base_url.copy_with(query=merged_query)
+            else:
+                self.url = base_url
         self.headers = Headers(headers)
         self.extensions = {} if extensions is None else dict(extensions)
 
diff --git a/tests/httpx2/client/test_queryparams.py b/tests/httpx2/client/test_queryparams.py
index c46780d3..e16cb453 100644
--- a/tests/httpx2/client/test_queryparams.py
+++ b/tests/httpx2/client/test_queryparams.py
@@ -31,3 +31,30 @@ def test_client_queryparams_echo():
 
     assert response.status_code == 200
     assert response.url == "http://example.org/echo_queryparams?first=str&second=dict"
+
+
+def test_base_url_with_request_params():
+    # Query params in the request URL must not be dropped when request-level
+    # params are also passed.
+    client = httpx2.Client(base_url="https://api.example.com/v1/")
+    request = client.build_request("GET", "users?active=true", params={"page": "2"})
+
+    assert str(request.url) == "https://api.example.com/v1/users?active=true&page=2"
+
+
+def test_base_url_with_client_params_and_url_query():
+    # Client-level params must be appended to query params already present in
+    # the request URL, not replace them.
+    client = httpx2.Client(base_url="https://api.example.com/v1/", params={"api_key": "abc"})
+    request = client.build_request("GET", "users?active=true")
+
+    assert str(request.url) == "https://api.example.com/v1/users?active=true&api_key=abc"
+
+
+def test_base_url_with_client_params_request_params_and_url_query():
+    # All three sources of query params (URL, client-level, request-level)
+    # must be combined without any being dropped.
+    client = httpx2.Client(base_url="https://api.example.com/v1/", params={"api_key": "abc"})
+    request = client.build_request("GET", "users?active=true", params={"page": "2"})
+
+    assert str(request.url) == "https://api.example.com/v1/users?active=true&api_key=abc&page=2"
diff --git a/tests/httpx2/models/test_requests.py b/tests/httpx2/models/test_requests.py
index 08be406a..d509672c 100644
--- a/tests/httpx2/models/test_requests.py
+++ b/tests/httpx2/models/test_requests.py
@@ -230,8 +230,16 @@ def test_request_params():
     request = httpx2.Request("GET", "http://example.com", params={})
     assert str(request.url) == "http://example.com"
 
+    # params are appended to, not replacing, any existing query in the URL
     request = httpx2.Request("GET", "http://example.com?c=3", params={"a": "1", "b": "2"})
-    assert str(request.url) == "http://example.com?a=1&b=2"
+    assert str(request.url) == "http://example.com?c=3&a=1&b=2"
 
     request = httpx2.Request("GET", "http://example.com?a=1", params={})
-    assert str(request.url) == "http://example.com"
+    assert str(request.url) == "http://example.com?a=1"
+
+
+def test_request_params_no_double_encoding():
+    # The existing query string must not be reparsed through QueryParams;
+    # doing so risks double-encoding or reordering of existing parameters.
+    request = httpx2.Request("GET", "http://example.com?q=hello%20world", params={"page": "2"})
+    assert str(request.url) == "http://example.com?q=hello%20world&page=2"