Monte Carlo integration methods employing a classic molecular mechanics force field were used to probe the impact of confinement on the selectivity of the n-hexane/3-methylpentane test reaction in 12 different zeolites. The simulations estimated relative cracking rates and predicted the constraint index (CI) selectivity for both monomolecular and bimolecular cracking mechanisms. By comparing with experimentally measured values of the CI, information on the dominant mechanism responsible for the observed selectivity was obtained. For small-pore zeolites, the monomolecular mechanism dominated with the measured CI attributed to reactant selectivity based on preferential adsorption. For large-pore zeolites, the CI selectivity was attributed to confinement effects imposed on the bimolecular transition states. For zeolite structures with intermediate-sized pores, such as MTW and TON, the simulations indicated that both reaction mechanisms may be operative. The experimentally observed reduction in the CI with temperature in ZSM-5 can be explained in terms of the relative importance of steric and entropic factors in the stabilization of the bimolecular transition state. A change in mechanism from bimolecular to monomolecular, as postulated in the literature, is not necessary to explain the experimentally observed temperature dependence of the CI.