--- rdf.api.inc	2008-12-23 12:21:10.000000000 +0100
+++ rdf.api.modified.inc	2008-12-23 12:51:38.000000000 +0100
@@ -785,10 +785,15 @@ function rdf_get_prefixes(array $data, $
 
   foreach ($data as $subject => $predicates) {
     foreach ($predicates as $predicate => $objects) {
-      $qname = rdf_uri_to_qname($predicate);
-      list($prefix, ) = explode(':', $qname);
-      if (!isset($result[$prefix])) {
-        $result[$prefix] = $namespaces[$prefix];
+      $qnames = array(rdf_uri_to_qname($predicate));
+      foreach($objects as $object) {
+        $qnames[] = $object->uri;
+      }
+      foreach($qnames as $qname) {
+        list($prefix, ) = explode(':', $qname);
+        if (!isset($result[$prefix])) {
+          $result[$prefix] = $namespaces[$prefix];
+        }
       }
     }
   }