diff --git a/features/ruby_gemfile.feature b/features/ruby_gemfile.feature
index 297ef91..8156346 100644
--- a/features/ruby_gemfile.feature
+++ b/features/ruby_gemfile.feature
@@ -249,3 +249,17 @@ Feature: Gemfiles
       # generated by penchant, environment: local
       gem "one", "1.2.3", {:path=>"../cats"}
       """
+
+  Scenario: Define special expansions for properties
+    Given I have the file "Gemfile.penchant" with the content:
+      """
+      defaults_for env(:local), :path => '../%s'
+      property(:github) { |name| { :git => "git://github.com/#{name}/%s.git" } }
+      gem 'one', :github => 'john'
+      """
+    When I rebuild the Gemfile for "local" mode
+    Then the file "Gemfile" should have the following content:
+      """
+      # generated by penchant, environment: local
+      gem "one", {:git=>"git://github.com/john/one.git"}
+      """
diff --git a/lib/penchant/gemfile.rb b/lib/penchant/gemfile.rb
index 4017a56..27b9f30 100644
--- a/lib/penchant/gemfile.rb
+++ b/lib/penchant/gemfile.rb
@@ -121,6 +121,7 @@ module Penchant
         @available_environments = []
         @defined_git_repos = []
         @defaults = {}
+        @properties = {}
 
         @_current_env_defaults = {}
       end
@@ -150,6 +151,10 @@ module Penchant
         end
       end
 
+      def property(name, &block)
+        @properties[name] = block
+      end
+
       def for_environment?(envs)
         envs.include?(environment) || environment == ANY_ENVIRONMENT
       end
@@ -192,13 +197,25 @@ module Penchant
       end
 
       def process_options(gem_name, template = {})
-        Hash[
-          _defaults_for(gem_name).dup.merge(template).collect { |key, value|
+        properties = {}
+
+        property_stack = _defaults_for(gem_name).dup.merge(template).to_a
+
+        while !property_stack.empty?
+          key, value = property_stack.shift
+
+          if @properties[key]
+            @properties[key].call(*([ value ].flatten)).each do |k, v|
+              property_stack.push([ k, v ])
+            end
+          else
             value = value % gem_name if value.respond_to?(:%)
 
-            [ key, value ]
-          }.sort
-        ]
+            properties[key] = value
+          end
+        end
+
+        Hash[properties.sort]
       end
 
       def _defaults_for(gem_name)