H- and P-t/H-forms of zeolites Y and mordenite were studied in atmospheric n-heptane (n-C-7) hydroconversion at 423-443 K. Adamantane (Ad) additive was found to increase the isomerization and suppress the cracking activity of the H-zeolites. It had minor or no influence on the hydroconversion activity of the Pt/H-forms. Species obtained from Ad adsorbed on zeolites were examined by IR spectroscopy. The Ad-saturated samples (Ad/H-zeolites) were boiled with n-C-7 and the liquid phase was analyzed by GC-MS. Results suggested that Ad was activated over the H-zeolites for further reactions via protolysis of its C-H bonds. The Ad and the formed adamantyl carbenium ions (Ad(+)) participated in hydride transfer processes. In these processes, the Ad remained unconverted and acted as co-catalyst increasing n-C-7 isomerization selectivity. Results suggested that under conditions Of n-C-7 hydroconversion no or minor amount of Ad(+) was generated on the PUH-zeolite catalysts. Over strong acid mordenite bulky carbocations and Ad compounds were rapidly formed contributing to the decay of the activity in n-C-7 conversion.