Aspects of the present invention relate to an improved process for exchanging sodium ions in zeolites comprising sodium ions and rare earth metal ions for ammonium ions. For this exchange, aqueous solutions of ammonium salts, for example ammonium sulfate, ammonium nitrate or ammonium chloride, are currently being used. The resulting “ammonium zeolites” are calcined to convert them, with release of ammonia, to the H form of the zeolites suitable as a catalyst. The use of ammonium carbonate also minimizes the amount of rare earth metal ions which are leached out of the zeolites comprising rare earth metal ions.