diff --git a/lib/sendgrid.rb b/lib/sendgrid.rb
index 7c68943..92d5e78 100644
--- a/lib/sendgrid.rb
+++ b/lib/sendgrid.rb
@@ -161,7 +161,12 @@ def sendgrid_ganalytics_options(options)
     @ganalytics_options = []
     options.each { |option| @ganalytics_options << option if VALID_GANALYTICS_OPTIONS.include?(option[0].to_sym) }
   end
-  
+
+  # Call within mailer method to set unsubscribe_group_id
+  def sendgrid_unsubscribe_group_id(unsubscribe_group_id)
+    @sg_asm_group_id = unsubscribe_group_id
+  end
+
   # only override the appropriate methods for the current ActionMailer version
   if ActionMailer::Base.respond_to?(:mail)
 
@@ -206,7 +211,7 @@ def sendgrid_json_headers(mail)
 
     #if not called within the mailer method, this will be nil so we default to empty hash
     @sg_unique_args = @sg_unique_args || {}
-    
+
     # set the unique arguments
     if @sg_unique_args || self.class.default_sg_unique_args
       unique_args = self.class.default_sg_unique_args || {}
@@ -254,6 +259,9 @@ def sendgrid_json_headers(mail)
       header_opts[:filters] = filters if filters && !filters.empty?
     end
 
+    # Set unsubscribe group
+    header_opts[:asm_group_id] = @sg_asm_group_id if @sg_asm_group_id
+
     header_opts.to_json.gsub(/(["\]}])([,:])(["\[{])/, '\\1\\2 \\3')
   end
 
diff --git a/sendgrid.gemspec b/sendgrid.gemspec
index ff125f0..a33430b 100644
--- a/sendgrid.gemspec
+++ b/sendgrid.gemspec
@@ -45,20 +45,17 @@ Gem::Specification.new do |s|
       s.add_development_dependency(%q<shoulda>, [">= 0"])
       s.add_development_dependency(%q<bundler>, ["~> 1.0.0"])
       s.add_development_dependency(%q<jeweler>, ["~> 1.5.1"])
-      s.add_runtime_dependency(%q<json>, [">= 0"])
     else
       s.add_dependency(%q<json>, [">= 0"])
       s.add_dependency(%q<shoulda>, [">= 0"])
       s.add_dependency(%q<bundler>, ["~> 1.0.0"])
       s.add_dependency(%q<jeweler>, ["~> 1.5.1"])
-      s.add_dependency(%q<json>, [">= 0"])
     end
   else
     s.add_dependency(%q<json>, [">= 0"])
     s.add_dependency(%q<shoulda>, [">= 0"])
     s.add_dependency(%q<bundler>, ["~> 1.0.0"])
     s.add_dependency(%q<jeweler>, ["~> 1.5.1"])
-    s.add_dependency(%q<json>, [">= 0"])
   end
 end