Abstract: The mechanism of n-hexane isomerization on mordenite which is used in a pilot plant as isomerization catalyst for light paraffin, was studied by "in situ" FT-IR study for reaction, evaluation of catalyst activity and kinetic analysis. The results show that the formation of carbonium ions can be explained by the existence of non-classical penta-coordinated carbonium ion intermediate. There may be an interpretation of a bimolecular cyclohexane-type mechanism for n-hexane isomerization on H-mordenite. A classical carbonium ion mechanism combined with a bimolecular intermediate mechanism may explain the isomerization of n-hexane on Pd/H-mordenite.