The activity and reaction paths for the hydroisomerization and hydrocracking of n-C-8 and various branched octane molecules over Pt/H-USY and Pt/H-ZSM-12 were studied. The relative sizes of the zeolite pore opening and the kinetic diameter of the molecule, as well as the degree of branching and position of the branches, all played a role in determining the rate of conversion of the molecule. Only n-C8 reacted at a faster rate over ZSM-12 than over USY; all the other isomers reacted faster over USY. The tribranched isomers 2,2,4-TMC5 and 2,3,4-TMC5 reacted several times faster than n-C8 over ZSM-12, and the formation of bulkier 2,2,3- and 2,3,3-TMC5 isomers was suppressed to a large extent, which indicated that the reaction was occurring inside the ZSM-12 pore. Even the distribution of the monobranched isomers was slightly affected by the shape selective effects in the ZSM-12 pore. Reactions that led to the removal of a branch from both monobranched and dibranched isomers were observed to a much larger extent over ZSM-12 than USY. These data, from catalytic reactions of molecules of various sizes, provide information about the true catalytic pore size of ZSM-12, which is greater than the crystallographic pore size. Over USY, 2,3,4-TMC5 reacted three times faster than 2,2,4-TMC5, even though it had to undergo an alkyl shift before the type A beta-scission step. This indicates that the reaction of 2,2,4-TMC5 is significantly slowed due to diffusion even over USY.