diff --git a/lib/compass-colors/sass_extensions.rb b/lib/compass-colors/sass_extensions.rb
index 33fb33d..77e10c6 100644
--- a/lib/compass-colors/sass_extensions.rb
+++ b/lib/compass-colors/sass_extensions.rb
@@ -4,28 +4,44 @@ module Sass::Script::Functions
   # Takes a color object and amount by which to lighten it (0 to 100).
   def lighten(color, amount)
     hsl = Compass::Colors::HSL.from_color(color)
-    hsl.l += (1 - hsl.l) * (amount.value / 100.0)
+    if percentage?(amount)
+      hsl.l += (1 - hsl.l) * (amount.value / 100.0)
+    else
+      hsl.l += amount.value
+    end
     hsl.to_color
   end
 
   # Takes a color object and amount by which to darken it (0 to 100).
   def darken(color, amount)
     hsl = Compass::Colors::HSL.from_color(color)
-    hsl.l *= 1.0 - (amount.value / 100.0)
+    if percentage?(amount)
+      hsl.l *= 1.0 - (amount.value / 100.0)
+    else
+      hsl.l -= amount.value
+    end
     hsl.to_color
   end
 
   # Saturate (make a color "richer") a color by the given amount (0 to 100)
   def saturate(color, amount)
     hsl = Compass::Colors::HSL.from_color(color)
-    hsl.s += (1 - hsl.s) * (amount.value / 100.0)
+    if percentage?(amount)
+      hsl.s += (1 - hsl.s) * (amount.value / 100.0)
+    else
+      hsl.s += amount.value
+    end
     hsl.to_color
   end
 
   # Desaturate (make a color "grayer") a color by the given amount (0 to 100)
   def desaturate(color, amount)
     hsl = Compass::Colors::HSL.from_color(color)
-    hsl.s *= (1.0 - (amount.value / 100.0))
+    if percentage?(amount)
+      hsl.s *= (1.0 - (amount.value / 100.0))
+    else
+      hsl.s -= amount.value
+    end
     hsl.to_color
   end
 
@@ -70,4 +86,10 @@ def complement(color)
     adjust_hue color, 180
   end
 
+  private
+
+  def percentage?(amount)
+    amount.numerator_units == ["%"] || (amount.unitless? && amount.value > 1 && amount.value < 100)
+  end
+
 end