Proton-catalyzed bimolecular reaction of isopropylbenzene with benzene leading to n-propylbenzene was studied employing ab initio quantum chemical calculations. The reaction complex was described by a complex of protonated isopropylbenzene and the benzene molecule. Geometry optimizations were performed at the Hartree-Fock level of theory under two conditions : (i) full relaxation of the geometrical parameters, reflecting the situation in nonrestricted reaction space; and (ii) imposing geometrical constraints, representing the steric conditions at the channel intersection in zeolites of the MFI and MEL structural types. Computations on the fully relaxed complex of protonated isopropylbenzene and benzene showed that n-propylbenzene formation cannot be expected on catalysts with nonrestricted reaction space. Use of the constrained model verified existence of a reaction pathway leading to n-propylbenzene formation, in agreement with the experimental observation.