The dehydrogenation reaction of isobutane over zeolites was investigated at the B3LYP/6-31G** and 6-311G** levels of calculation, and with T3 and T5 clusters representing the zeolite. The transition state (TS) exhibits a carbenium ion-like character, and the activation energy, at the best level of theory, is 53.4 kcal/mol. Contrary to what has been previously proposed, IRC calculations show that the mechanism does not involve the formation of alkoxide, but rather the carbocation collapses directly into isobutene while the eliminated proton, restores the zeolite's acid site. Increasing the size of the cluster and of the basis set does not change the mechanism.