diff --git a/lib/rscons/environment.rb b/lib/rscons/environment.rb
index b73ef35..b2f8f3a 100644
--- a/lib/rscons/environment.rb
+++ b/lib/rscons/environment.rb
@@ -6,7 +6,7 @@ module Rscons
   # contains a collection of construction variables, options, builders, and
   # rules for building targets.
   class Environment
-    # [Array] of {Builder} objects.
+    # [Hash] of {"builder_name" => builder_object} pairs.
     attr_reader :builders
 
     # Create an Environment object.
@@ -53,7 +53,7 @@ module Rscons
       @builders.each do |builder_name, builder|
         env.add_builder(builder)
       end
-      env.append(@varset)
+      env.append(@varset.clone)
       env.append(variables)
 
       if block_given?
diff --git a/spec/rscons/environment_spec.rb b/spec/rscons/environment_spec.rb
index 2e0d6dc..a362a8c 100644
--- a/spec/rscons/environment_spec.rb
+++ b/spec/rscons/environment_spec.rb
@@ -1,5 +1,16 @@
 module Rscons
   describe Environment do
+    describe '.clone' do
+      it 'should create unique copies of each construction variable' do
+        env = Environment.new
+        env["CPPPATH"] << "path1"
+        env2 = env.clone
+        env2["CPPPATH"] << "path2"
+        env["CPPPATH"].should == ["path1"]
+        env2["CPPPATH"].should == ["path1", "path2"]
+      end
+    end
+
     describe '.parse_makefile_deps' do
       it 'handles dependencies on one line' do
         File.should_receive(:read).with('makefile').and_return(<<EOS)