n-Butane transformation was investigated in a flow reactor at 523 K over a series of H-mordenite samples with Si/Al ratios of 6.6, 10, 20, 60, and 80, Under nitrogen, butane isomerization is accompanied by the formation in significant amounts of propane and pentanes. The reaction order with respect to n-butane is close to 2 for isobutane, for propane, and for pentane formation. These observations are in favor of a bimolecular mode of n-butane transformation involving successively formation of C-8 carbenium ions, isomerization, and cracking of these C-8 carbenium ions, A large positive effect of the density of acid sites is found, which can be related to the demanding character of this bimolecular process : two acid sites are required for the formation of the two sec-butyl carbenium ions involved as intermediates, Hydrogen strongly inhibits the transformation of n-butane, which can be explained by a decrease in the concentration of sec-butyl carbenium ions, An increase in the selectivity to isobutane and a decrease in the reaction order are observed, which seems to indicate participation of the very difficult monomolecular process (through the very unstable isobutyl carbenium ion) in butane isomerization.