The H/D exchange of CH4 with D-2 has been studied by on-line mass spectrometry over a series of transition metal/zeolite catalysts. Isotope exchange of CH4 with D-2 is rapid over reduced Pd/MFI under conditions where no exchange can be detected over H/MFI, Cu/MFI or Fe/MFI. The exchange mechanism is stepwise, deuteromethanes are formed in ratios predicted by statistical thermodynamics. This reaction can be used as a catalytic probe to detect the presence of Pd-n(0) clusters. It shows that in the selective reduction of NO over Pd/MFI some Pd-0 exists even in the presence of excess O-2. On those Pd/MFI catalysts where Pd-n(0) particles and acidic protons coexist, the exchange of surface hydrons against D-2 is strongly catalyzed by Pd-0, temperature programmed reaction identifies two types of sites with different rate constants.