The H/D exchange was studied between CD4 and H-zeolites using pulsed a microcatalytic reactor and MS analysis. Methane-d(3) was the only exchange product, while a large fraction of the reactant was retained by the zeolite. The apparent activation energies (E-a) were determined for CHD3 production assuming first-order reaction kinetics. The E-a was close to 80 kJ mol(-1) over the H-ZSM-5 and H-mordenite, and about 50 kJ mol(-1) over H-beta. Results suggested that, at the low methane loading applied, the H/D exchange was initiated predominantly by heterolytic dissociation of CD4 to CD3delta- and Ddelta+ over Lewis acid-Lewis base pair sites.