diff --git a/chrome/content/zotero/xpcom/translation/translate_item.js b/chrome/content/zotero/xpcom/translation/translate_item.js
index 2a3377d32..8cff06292 100644
--- a/chrome/content/zotero/xpcom/translation/translate_item.js
+++ b/chrome/content/zotero/xpcom/translation/translate_item.js
@@ -385,8 +385,13 @@ Zotero.Translate.ItemSaver.prototype = {
 				// try to map from base field
 				if(Zotero.ItemFields.isBaseField(fieldID)) {
 					fieldID = Zotero.ItemFields.getFieldIDFromTypeAndBase(typeID, fieldID);
+					
+					// Skip mapping if item field already exists
+					var fieldName = Zotero.ItemFields.getName(fieldID);
+					if(item[fieldName]) continue;
+					
 					if(fieldID) {
-						Zotero.debug("Translate: Mapping "+field+" to "+Zotero.ItemFields.getName(fieldID), 5);	
+						Zotero.debug("Translate: Mapping "+field+" to "+fieldName, 5);	
 					}
 				}
 				
diff --git a/chrome/content/zotero/xpcom/utilities.js b/chrome/content/zotero/xpcom/utilities.js
index e64a8fcde..ae17c3545 100644
--- a/chrome/content/zotero/xpcom/utilities.js
+++ b/chrome/content/zotero/xpcom/utilities.js
@@ -1262,7 +1262,9 @@ Zotero.Utilities = {
 				
 				// map from base field if possible
 				if((itemFieldID = Zotero.ItemFields.getFieldIDFromTypeAndBase(typeID, fieldID))) {
-					newItem[Zotero.ItemFields.getName(itemFieldID)] = val;
+					var fieldName = Zotero.ItemFields.getName(itemFieldID);
+					// Only map if item field does not exist
+					if(!newItem[fieldName]) newItem[fieldName] = val;
 					continue;	// already know this is valid
 				}