diff --git a/library/HTMLPurifier/Encoder.php b/library/HTMLPurifier/Encoder.php
index 40a24266..d4791cc1 100644
--- a/library/HTMLPurifier/Encoder.php
+++ b/library/HTMLPurifier/Encoder.php
@@ -398,8 +398,8 @@ class HTMLPurifier_Encoder
             // characters to their true byte-wise ASCII/UTF-8 equivalents.
             $str = strtr($str, self::testEncodingSupportsASCII($encoding));
             return $str;
-        } elseif ($encoding === 'iso-8859-1') {
-            $str = utf8_encode($str);
+        } elseif ($encoding === 'iso-8859-1' && function_exists('mb_convert_encoding')) {
+            $str = mb_convert_encoding($str, 'UTF-8', 'ISO-8859-1');
             return $str;
         }
         $bug = HTMLPurifier_Encoder::testIconvTruncateBug();
@@ -450,8 +450,8 @@ class HTMLPurifier_Encoder
             // Normal stuff
             $str = self::iconv('utf-8', $encoding . '//IGNORE', $str);
             return $str;
-        } elseif ($encoding === 'iso-8859-1') {
-            $str = utf8_decode($str);
+        } elseif ($encoding === 'iso-8859-1' && function_exists('mb_convert_encoding')) {
+            $str = mb_convert_encoding($str, 'ISO-8859-1', 'UTF-8');
             return $str;
         }
         trigger_error('Encoding not supported', E_USER_ERROR);