Molecular sieves are modified by an ion exchange process where metal cations are introduced into the molecular sieves by means of solid state ion exchange. The solid state ion exchange can be carried out as follows: a weighed amount of calcined and activated zeolite is intimately mixed with a precalculated amount of PtCl.sub.2, PdCl.sub.2, RhCl.sub.3, CuCl.sub.2, V.sub.2 O.sub.5 or another compound of the noble metals (e.g., corresponding halides or oxides), the solids mixture is then heated in a current of inert gas (e.g., a current of helium gas or of nitrogen) to temperatures of 400.degree. to 600.degree. C., then cooled down to room temperature and subsequently reduced in a current of hydrogen for 10 to 14 hours at 280.degree. to 350.degree. C. in order to produce small metal clusters from the cationically introduced metal.