The mechanism of n-propyltoluene formation in toluene alkylation with propanols has been investigated over molecular sieves of H-ZSM-5, H-Y, and H-mordenite structures. It has been shown that isopropyltoluenes are formed in the first alkylation step followed by a bimolecular reaction with toluene molecules leading to n-propyltoluenes. This mechanism is evidenced by the reaction of p-isopropyltoluene with benzene and isopropylbenzene with toluene leading to n-propylbenzene and n-propyltoluene, respectively. The bimolecular isomerization reaction takes place only in the three-dimensional channel system of H-ZSM-5 and H-Y, enabling the transition complex formation, which is enhanced by the channel geometry of H-ZSM-5. Therefore, "structure-directed transition state selectivity," controlled by the molecular sieve structure, operates in this isomerization reaction.