The skeletal isomerization of a 2-pentene mol. catalyzed by acidic ZSM-22 was studied by ab initio DFT studies. Two different scenarios proposed in the literature were tested. First a reaction including an alkyl shift was considered: a Me or Et group is detached from the carbenium ion chain and reattached at another site in the residual hydrocarbon chain. However, this mechanism is rather unlikely, since the alkyl ion is a high-energy species, so its detachment from the carbenium ion induces a high activation energy. The more likely pathway for skeletal isomerization inside the channels of ZSM-22 involves the rearrangement of the carbenium ion into a protonated dimethylcyclopropane and implies the formation of relatively stable secondary carbenium ions as transient intermediates.