Over HMOR zeolites, hydrogen inhibits n-butane isomerization which occurs through a bimolecular pathway and has practically no effect on n-hexane isomerization, of which the reaction mechanism is intramolecular. The large inhibiting effect found with butane isomerization is certainly related to the demanding character of the bimolecular process : two sec-butyl carbenium ions are necessary for the alkylation step. Hydrogen could react with carbenium ions limiting their concentration. Over sulfated zirconia hydrogen has also an inhibiting effect on n-butane isomerization, this effect being particularly pronounced at low temperature, and has no effect on n-hexane isomerization. It is suggested that the differences in the butane isomerization mechanisms proposed in the literature in the case of sulfated zirconia are mainly due to the "diluent" gas used (nitrogen and hydrogen) and for a small part to the reaction temperature.