diff --git a/features/ruby_gemfile.feature b/features/ruby_gemfile.feature
index f9391e8..ba5a032 100644
--- a/features/ruby_gemfile.feature
+++ b/features/ruby_gemfile.feature
@@ -129,3 +129,26 @@ Feature: Gemfiles
       gem "one", {:path=>"../one"}
       """
 
+  @wip @mocha
+  Scenario: OS-specific blocks
+    Given I have the file "Gemfile.penchant" with the content:
+      """
+      os :darwin do
+        gem 'one', :path => '../%s'
+      end
+      """
+    And I am on the "darwin" platform
+    When I rebuild the Gemfile for "local" mode
+    Then the file "Gemfile" should have the following content:
+      """
+      # generated by penchant, environment: local
+      gem "one", {:path=>"../one"}
+      """
+    Given I am on the "linux" platform
+    When I rebuild the Gemfile for "local" mode
+    Then the file "Gemfile" should have the following content:
+      """
+      # generated by penchant, environment: local
+
+      """
+
diff --git a/features/step_definitions/given/i_am_on_the_linux_platform.rb b/features/step_definitions/given/i_am_on_the_linux_platform.rb
new file mode 100644
index 0000000..c8bd68b
--- /dev/null
+++ b/features/step_definitions/given/i_am_on_the_linux_platform.rb
@@ -0,0 +1,3 @@
+Given /^I am on the "([^"]*)" platform$/ do |os|
+  Penchant::Gemfile::PenchantFile.any_instance.stubs(:current_os).returns(os.to_sym)
+end
diff --git a/features/support/env.rb b/features/support/env.rb
index caee814..6f8f552 100644
--- a/features/support/env.rb
+++ b/features/support/env.rb
@@ -1,14 +1,27 @@
 require 'fakefs/safe'
 require 'penchant'
+require 'mocha'
+
+World(Mocha::Standalone)
 
 Before('@fakefs') do
   FakeFS.activate!
 end
 
+Before('@mocha') do
+  mocha_setup
+end
+
 After do
   FakeFS::FileSystem.clear
   FakeFS.deactivate!
 
+  begin
+    mocha_verify
+  ensure
+    mocha_teardown
+  end
+
   FileUtils.rm_rf 'tmp'
 end
 
diff --git a/lib/penchant/gemfile.rb b/lib/penchant/gemfile.rb
index 4e504f6..cba2c99 100644
--- a/lib/penchant/gemfile.rb
+++ b/lib/penchant/gemfile.rb
@@ -112,6 +112,10 @@ module Penchant
         yield if !is_deployment
       end
 
+      def os(*args)
+        yield if args.include?(current_os)
+      end
+
       protected
       def args_to_string(args)
         args.inspect[1..-2]
@@ -144,6 +148,18 @@ module Penchant
           }.sort
         ]
       end
+
+      def current_os
+        require 'rbconfig'
+        case host_os = RbConfig::CONFIG['host_os']
+        when /darwin/
+          :darwin
+        when /linux/
+          :linux
+        else
+          host_os[%r{^[a-z]+}, 1].to_sym
+        end
+      end
     end
 
     class ERBFile < FileProcessor