Ab initio calculations showing frontier molecular orbital energy modifications of a guest molecule when located inside a microporous zeolitic cavity are presented. Micropores of zeolites Beta, ZSM-12, and ZSM-5 have been modeled using all silica clusters from which it is found that the highest occupied molecular orbital (HOMO) energy of toluene increases when going from the gas phase state to restricted microporous environments. The influence of the zeolite cavity size and its chemical composition on the toluene HOMO energy are discussed. Regarding the size of the confining environment, it is found that the smaller the zeolite cavity the higher is the rise in toluene HOMO energy. The effect of the chemical composition on the toluene HOMO energy was tested in the ZSM-5 zeolite by varying the Al content. The results obtained showed that the frontier orbital energy increases upon decreasing the Al content of the cluster. As a consequence of the confinement effect, toluene reactivity in zeolite catalyzed reactions is expected to change toward a more covalent behavior in which electronic transference from the toluene molecule to an electronic acceptor will be more favored than in gas phase reactions.