diff --git a/chrome/content/zotero/xpcom/integration.js b/chrome/content/zotero/xpcom/integration.js
index f42cabb6ac..95b6a681c4 100644
--- a/chrome/content/zotero/xpcom/integration.js
+++ b/chrome/content/zotero/xpcom/integration.js
@@ -990,7 +990,7 @@ Zotero.Integration.Document.prototype._getSession = Zotero.Promise.coroutine(fun
 		return this._session.setDocPrefs(this._doc, this._app.primaryFieldType,
 		this._app.secondaryFieldType).then(function(status) {
 			// save doc prefs in doc
-			me._doc.setDocumentData(me._session.data.serializeXML());
+			me._doc.setDocumentData(me._session.data.serialize());
 			
 			if(haveFields) {
 				me._session.reload = true;
@@ -1007,12 +1007,13 @@ Zotero.Integration.Document.prototype._getSession = Zotero.Promise.coroutine(fun
 				data.prefs.fieldType = "ReferenceMark";
 			}
 			
-			var warning = this._doc.displayAlert(Zotero.getString("integration.upgradeWarning"),
+			var warning = this._doc.displayAlert(Zotero.getString("integration.upgradeWarning", Zotero.clientName, '5.0'),
 				DIALOG_ICON_WARNING, DIALOG_BUTTONS_OK_CANCEL);
 			if(!warning) {
 				return Zotero.Promise.reject(new Zotero.Exception.UserCancelled("document upgrade"));
 			}
-		} else if(data.dataVersion > DATA_VERSION) {
+		// Don't throw for version 4(JSON) during the transition from 4.0 to 5.0
+		} else if((data.dataVersion > DATA_VERSION) && data.dataVersion != 4) {
 			return Zotero.Promise.reject(new Zotero.Exception.Alert("integration.error.newerDocumentVersion",
 					[data.zoteroVersion, Zotero.version], "integration.error.title"));
 		}
@@ -1046,7 +1047,7 @@ Zotero.Integration.Document.prototype._getSession = Zotero.Promise.coroutine(fun
 					}
 					return this._session.setDocPrefs(this._doc, this._app.primaryFieldType,
 					this._app.secondaryFieldType).then(function(status) {			
-						me._doc.setDocumentData(me._session.data.serializeXML());
+						me._doc.setDocumentData(me._session.data.serialize());
 						me._session.reload = true;
 						return me._session;
 					});
@@ -1259,7 +1260,7 @@ Zotero.Integration.Document.prototype.setDocPrefs = function() {
 		oldData = aOldData;
 		
 		// Write document data to document
-		me._doc.setDocumentData(me._session.data.serializeXML());
+		me._doc.setDocumentData(me._session.data.serialize());
 		
 		// If oldData is null, then there was no document data, so we don't need to update
 		// fields
@@ -1736,7 +1737,7 @@ Zotero.Integration.Fields.prototype._updateDocument = function* (forceCitations,
 				
 				// set bibliographyStyleHasBeenSet parameter to prevent further changes	
 				this._session.data.style.bibliographyStyleHasBeenSet = true;
-				this._doc.setDocumentData(this._session.data.serializeXML());
+				this._doc.setDocumentData(this._session.data.serialize());
 			}
 		}
 		
@@ -3080,9 +3081,20 @@ Zotero.Integration.DocumentData = function(string) {
 }
 
 /**
- * Serializes document-specific data as XML
+ * Serializes document-specific data as JSON
  */
-Zotero.Integration.DocumentData.prototype.serializeXML = function() {
+Zotero.Integration.DocumentData.prototype.serialize = function() {
+	// If we've retrieved data with version 4 (JSON), serialize back to JSON
+	if (this.dataVersion == 4) {
+		return JSON.stringify({
+			style: this.style,
+			prefs: this.prefs,
+			sessionID: this.sessionID,
+			zoteroVersion: Zotero.version,
+			dataVersion: 4
+		});
+	}
+	// Otherwise default to XML for now
 	var prefs = "";
 	for(var pref in this.prefs) {
 		prefs += `<pref name="${Zotero.Utilities.htmlSpecialChars(pref)}" `+
@@ -3136,7 +3148,14 @@ Zotero.Integration.DocumentData.prototype.unserializeXML = function(xmlData) {
  * Unserializes document-specific data, either as XML or as the string form used previously
  */
 Zotero.Integration.DocumentData.prototype.unserialize = function(input) {
-	if(input[0] == "<") {
+	try {
+		return Object.assign(this, JSON.parse(input))
+	} catch (e) {
+		if (!(e instanceof SyntaxError)) {
+			throw e;
+		}
+	}
+	if (input[0] == "<") {
 		this.unserializeXML(input);
 	} else {
 		const splitRe = /(^|[^:]):(?!:)/;
diff --git a/chrome/locale/en-US/zotero/zotero.properties b/chrome/locale/en-US/zotero/zotero.properties
index 855709c9aa..9f9be2452e 100644
--- a/chrome/locale/en-US/zotero/zotero.properties
+++ b/chrome/locale/en-US/zotero/zotero.properties
@@ -832,7 +832,7 @@ integration.missingItem.single			= The highlighted citation no longer exists in
 integration.missingItem.multiple		= Item %1$S in the highlighted citation no longer exists in your Zotero database. Do you want to select a substitute item?
 integration.missingItem.description		= Clicking "No" will delete the field codes for citations containing this item, preserving the citation text but deleting it from your bibliography.
 integration.removeCodesWarning			= Removing field codes will prevent Zotero from updating citations and bibliographies in this document. Are you sure you want to continue?
-integration.upgradeWarning				= Your document must be permanently upgraded in order to work with Zotero 2.1 or later. It is recommended that you make a backup before proceeding. Are you sure you want to continue?
+integration.upgradeWarning				= Your document must be permanently upgraded in order to work with %S %S or later. It is recommended that you make a backup before proceeding. Are you sure you want to continue?
 integration.error.newerDocumentVersion			= Your document was created with a newer version of Zotero (%1$S) than the currently installed version (%2$S). Please upgrade Zotero before editing this document.
 integration.corruptField				= The Zotero field code corresponding to this citation, which tells Zotero which item in your library this citation represents, has been corrupted. Would you like to reselect the item?
 integration.corruptField.description	= Clicking "No" will delete the field codes for citations containing this item, preserving the citation text but potentially deleting it from your bibliography.
diff --git a/test/tests/integrationTests.js b/test/tests/integrationTests.js
index 808c1ccff7..05a9598601 100644
--- a/test/tests/integrationTests.js
+++ b/test/tests/integrationTests.js
@@ -255,7 +255,7 @@ describe("Zotero.Integration", function () {
 		data.style = {styleID, locale: 'en-US', hasBibliography: true, bibliographyStyleHasBeenSet: true};
 		data.sessionID = Zotero.Utilities.randomString(10);
 		Object.assign(data, options);
-		applications[docID].getActiveDocument().setDocumentData(data.serializeXML());
+		applications[docID].getActiveDocument().setDocumentData(data.serialize());
 	}
 	
 	function setDefaultIntegrationDocPrefs() {
@@ -472,7 +472,7 @@ describe("Zotero.Integration", function () {
 	});
 	
 	describe("DocumentData", function() {
-		it('should properly unserialize document data', function() {
+		it('should properly unserialize old XML document data', function() {
 			var serializedXMLData = "<data data-version=\"3\" zotero-version=\"5.0.SOURCE\"><session id=\"F0NFmZ32\"/><style id=\"http://www.zotero.org/styles/cell\" hasBibliography=\"1\" bibliographyStyleHasBeenSet=\"1\"/><prefs><pref name=\"fieldType\" value=\"ReferenceMark\"/><pref name=\"storeReferences\" value=\"true\"/><pref name=\"automaticJournalAbbreviations\" value=\"true\"/><pref name=\"noteType\" value=\"0\"/></prefs></data>";
 			var data = new Zotero.Integration.DocumentData(serializedXMLData);
 			var expectedData = {
@@ -496,7 +496,30 @@ describe("Zotero.Integration", function () {
 			assert.equal(JSON.stringify(data), JSON.stringify(expectedData));
 		});
 		
-		it('should properly serialize document data', function() {
+		it('should properly unserialize JSON document data', function() {
+			var expectedData = JSON.stringify({
+				style: {
+					styleID: 'http://www.zotero.org/styles/cell',
+					locale: 'en-US',
+					hasBibliography: true,
+					bibliographyStyleHasBeenSet: true
+				},
+				prefs: {
+					fieldType: 'ReferenceMark',
+					storeReferences: true,
+					automaticJournalAbbreviations: false,
+					noteType: 0
+				},
+				sessionID: 'owl-sesh',
+				zoteroVersion: '5.0.SOURCE',
+				dataVersion: 4
+			});
+			var data = new Zotero.Integration.DocumentData(expectedData);
+			// Convert to JSON to remove functions from DocumentData object
+			assert.equal(JSON.stringify(data), expectedData);
+		});
+		
+		it('should properly serialize document data to XML (data ver 3)', function() {
 			sinon.spy(Zotero, 'debug');
 			var data = new Zotero.Integration.DocumentData();
 			data.sessionID = "owl-sesh";
@@ -515,8 +538,11 @@ describe("Zotero.Integration", function () {
 				automaticJournalAbbreviations: true
 			};
 			
+			var serializedData = data.serialize();
+			// Make sure we serialized to XML here
+			assert.equal(serializedData[0], '<');
 			// Serialize and unserialize (above test makes sure unserialize works properly).
-			var processedData = new Zotero.Integration.DocumentData(data.serializeXML());
+			var processedData = new Zotero.Integration.DocumentData(serializedData);
 			
 			// This isn't ideal, but currently how it works. Better serialization which properly retains types
 			// coming with official 5.0 release.
@@ -530,5 +556,34 @@ describe("Zotero.Integration", function () {
 			assert.isFalse(Zotero.debug.calledWith(sinon.match.string, 1));
 			Zotero.debug.restore();
 		});
+		
+		it('should properly serialize document data to JSON (data ver 4)', function() {
+			var data = new Zotero.Integration.DocumentData();
+			// data version 4 triggers serialization to JSON
+			// (e.g. when we've retrieved data from the doc and it was ver 4 already)
+			data.dataVersion = 4;
+			data.sessionID = "owl-sesh";
+			data.style = {
+				styleID: 'http://www.zotero.org/styles/cell',
+				locale: 'en-US',
+				hasBibliography: false,
+				bibliographyStyleHasBeenSet: true
+			};
+			data.prefs = {
+				noteType: 1,
+				fieldType: "Field",
+				storeReferences: true,
+				automaticJournalAbbreviations: true
+			};
+			
+			// Serialize and unserialize (above tests makes sure unserialize works properly).
+			var processedData = new Zotero.Integration.DocumentData(data.serialize());
+
+			// Added in serialization routine
+			data.zoteroVersion = Zotero.version;
+			
+			// Convert to JSON to remove functions from DocumentData objects
+			assert.deepEqual(JSON.parse(JSON.stringify(processedData)), JSON.parse(JSON.stringify(data)));
+		});
 	})
 });