diff --git a/lib/sass/constant/literal.rb b/lib/sass/constant/literal.rb
index 72958e8f..c4ee849c 100644
--- a/lib/sass/constant/literal.rb
+++ b/lib/sass/constant/literal.rb
@@ -6,7 +6,7 @@ require 'sass/constant/color'
 
 class Sass::Constant::Literal
   # The regular expression matching numbers.
-  NUMBER  = /^\-?[0-9]*\.?[0-9]+$/
+  NUMBER  = /^(-?[0-9]*?\.?)([0-9]+)([^0-9]*)$/
 
   # The regular expression matching colors.
   COLOR = /^\#(#{"[0-9a-fA-F]" * 3}|#{"[0-9a-fA-F]" * 6})/
diff --git a/lib/sass/constant/number.rb b/lib/sass/constant/number.rb
index 5b561b7f..4f1b65c3 100644
--- a/lib/sass/constant/number.rb
+++ b/lib/sass/constant/number.rb
@@ -2,10 +2,13 @@ require 'sass/constant/literal'
 
 module Sass::Constant
   class Number < Literal
-  
+
+    attr_reader :unit
+
     def parse(value)
-      value = value.include?('.') ? value.to_f : value.to_i
-      @value = value
+      first, second, unit = value.scan(Literal::NUMBER)[0]
+      @value = first.empty? ? second.to_i : "#{first}#{second}".to_f
+      @unit = unit unless unit.empty?
     end
     
     def plus(other)
@@ -55,13 +58,30 @@ module Sass::Constant
     def to_s
       value = @value
       value = value.to_i if value % 1 == 0.0
-      value.to_s
+      "#{value}#{@unit}"
     end
     
     protected
+
+    def self.from_value(value, unit=nil)
+      instance = super(value)
+      instance.instance_variable_set('@unit', unit)
+      instance
+    end
     
     def operate(other, operation)
-      Number.from_value(self.value.send(operation, other.value))
+      unit = nil
+      if other.unit.nil?
+        unit = self.unit
+      elsif self.unit.nil?
+        unit = other.unit
+      elsif other.unit == self.unit
+        unit = self.unit
+      else
+        raise Sass::SyntaxError.new("Incompatible units: #{self.unit} and #{other.unit}")
+      end
+
+      Number.from_value(self.value.send(operation, other.value), unit)
     end
   end
 end
diff --git a/test/sass/engine_test.rb b/test/sass/engine_test.rb
index af1bc6c4..1b0c64d2 100644
--- a/test/sass/engine_test.rb
+++ b/test/sass/engine_test.rb
@@ -32,7 +32,8 @@ class SassEngineTest < Test::Unit::TestCase
     "a\n\t:b c" => "Illegal Indentation: Only two space characters are allowed as tabulation.",
     "a\n :b c" => "Illegal Indentation: Only two space characters are allowed as tabulation.",
     "a\n    :b c" => "Illegal Indentation: Only two space characters are allowed as tabulation.",
-    "a\n  :b\n  !c = 3" => "Constants may only be declared at the root of a document."
+    "a\n  :b\n  !c = 3" => "Constants may only be declared at the root of a document.",
+    "!a = 1b + 2c" => "Incompatible units: b and c"
   }
   
   def test_basic_render
diff --git a/test/sass/results/constants.css b/test/sass/results/constants.css
index d6cf3466..5fe3f639 100644
--- a/test/sass/results/constants.css
+++ b/test/sass/results/constants.css
@@ -1,7 +1,7 @@
-#main { content: Hello!; qstr: Quo"ted"!; hstr: Hyph-en!; width: 30; background-color: #000; color: #ffffaf; con: foo bar 9 hi there boom; }
+#main { content: Hello!; qstr: Quo"ted"!; hstr: Hyph-en!; width: 30em; background-color: #000; color: #ffffaf; con: foo bar 9 hi there boom; }
 #main #sidebar { background-color: #00ff98; num-normal: 10; num-dec: 10.2; num-dec0: 99; num-neg: -10; esc: 10+12; many: 6; order: 7; complex: #4c9db1hi16; }
 
-#plus { num-num: 7; num-num-neg: -3; num-str: 100px; num-col: #bcbcbc; str-str: hi there; str-str2: hi there; str-num: times: 13; col-num: #ff8aaa; col-col: #5f80ff; }
+#plus { num-num: 7; num-num-un: 25em; num-num-neg: 9.87; num-str: 100px; num-col: #bcbcbc; str-str: hi there; str-str2: hi there; str-num: times: 13; col-num: #ff8aaa; col-col: #5f80ff; }
 
 #minus { num-num: 900; col-num: #f9f9f4; col-col: #000035; }
 
diff --git a/test/sass/templates/constants.sass b/test/sass/templates/constants.sass
index 6fdd0161..ad886f7b 100644
--- a/test/sass/templates/constants.sass
+++ b/test/sass/templates/constants.sass
@@ -1,4 +1,4 @@
-!width = 10 + 20
+!width = 10em + 20
 !color = #00ff98
 !main_text = #ffa
 !num = 10
@@ -34,7 +34,8 @@
 #plus
   :num
     :num= 5+2
-    :num-neg= 10 + -13
+    :num-un= 10em + 15em
+    :num-neg= 10 + -.13
     :str= 100 + px
     :col= 13 + #aaa
   :str