diff --git a/cssmin/Minifier.php b/cssmin/Minifier.php
index ccbf6e5..a09e2bc 100644
--- a/cssmin/Minifier.php
+++ b/cssmin/Minifier.php
@@ -326,6 +326,13 @@ private function minify($css)
             $css
         );
 
+        // Process mathematical expressions so their operators don't get accidentally minified
+        $css = preg_replace_callback(
+            '/\b\d*\.?\d+[a-z%]*\s*[\+\-\*\/]\s*\d*\.?\d+[a-z%]*\b/Si',
+            array($this, 'processMathematicalExpressionCallback'),
+            $css
+        );
+
         // Normalize all whitespace strings to single spaces. Easier to work with that way.
         $css = preg_replace('/\s+/S', ' ', $css);
         
@@ -454,6 +461,16 @@ private function processStringsCallback($matches)
         return $quote . $this->registerPreservedToken($match) . $quote;
     }
 
+    /**
+     * Preserves mathematical expressions found
+     * @param array $matches
+     * @return string
+     */
+    private function processMathematicalExpressionCallback($matches)
+    {
+        return $this->registerPreservedToken($matches[0]);
+    }
+
     /**
      * Preserves or removes comments found.
      * @param string $css