We carry out experimental hydroconversion tests of n-heptane paraffin on five different zeolites: Y, beta, ZSM-22, ZSM-23, and EU-1, The experimental selectivities clearly depend on the catalyst pore structures. Through force field simulations, we investigate adsorption and diffusion properties as well as model cyclopropane intermediate stabilities inside each zeolite framework. In open structures such as Y and beta, no shape transition state selectivity or diffusion limitation are taking place. In the more restricted 10-membered ring (MR) channels of ZSM-22, ZSM-23, and 10-MR windows of EU-1, we show that the selectivities in branched isomers and cracked products are explained by the combined effects of transition state restriction and product diffusion limitations. In particular, the selectivities in monomethyl-branched products are correlated to their relative diffusion barriers, in agreement with earlier results of E. B. Webb III et al. (1999, J. Phys. Chem. B 103, 4949). The explanation for this result has its foundation in the degree of symmetry of the monomethyl-branched paraffin. In ZSM-22 and ZSM-23, the 2,2- and 3,3-dMC(5) are not formed on account of transition-state shape restrictions, whereas a slight amount of 2,3- and 2,4-dMC(5) is formed and can diffuse out of the pores. In the EU-1 sieve, the large side pockets behave as the open Y or B structures, where all isomerization reactions can take place without restriction, but only the faster diffusing 2,3-dMC(5) and 2,4-dMC(5) products are released. The slower diffusing multibranched 2,2-dMC(5) and 3,3-dMC(5) paraffins are transformed via methyl shift or cracked inside the pockets producing a higher amount of cracked products in EU-1 than in ZSM-22 or ZSM-23. (C) 2001 Academic Press.