Abstract Formation of unsaturated carbenium ions from 1-hexene, cyclohexane, cyclohexene, cyclohexadiene and benzene upon interaction with the H-forms of zeolites ZSM-5 and γ was proved by UV-VIS and IR spectroscopy. With the exception of benzene, which forms dienyl ions at the beginning, carbocation formation from the other C6 compounds starts with monoenylic species, which transform into oligoenylic ions with time of contact. From the cyclic hydrocarbons cyclic ions as well as open-chain alkenyl ions are formed. The ion formation capability decreases in the sequence cyclohexadiene > cyclohexene > 1-hexene > cyclohexane > benzene. For the first stages of carbenium ion development the formation of aromatic surface species may be excluded.