Ammonium zeolites of extremely low alkali metal content are prepared by a process of potassium ion exchange followed by ammonium ion exchange. Zeolite X or zeolite Y that contain a significant amount of sodium are contacted with a potassium salt solution under conditions that provide a substantial exchange of potassium for sodium. The potassium enriched zeolite is then contacted with an ammonium salt solution so that the ammonium ion replaces the sodium and potassium ions. The resulting ammonium zeolite X or Y contains considerably less than 1% alkali metal calculated as Na.sub.2 O.