This study addresses the influence of USY zeolite crystallite size on catalyst activity and selectivity during the catalytic conversion of methylcyclohexane (MCH). The crystallite size of the supported zeolites is varied between 0.4 and 0.9 mu m with both activity and selectivity being monitored. Experiments are developed in a fluidized bench-scale batch CREC riser simulator reactor. MCH overall conversion ranges between 4 and 16 wt.%, with higher conversions obtained using the larger zeolite crystallites due to their slightly higher acidity. It is found that even if the difference in zeolite crystallite size does not have a significant effect on MCH conversion; it still is an important factor affecting product distribution. Bi-molecular reactions between adsorbed species in neighboring sites, particularly hydrogen transfer, are promoted using the 0.9 mu m zeolite crystallites. This is assigned to the increased opportunity for adsorbed chemical species to interact with each other within the bigger surface areas per crystallite and longer pores provided by the larger crystallites. To provide quantifications of the differences in activity and selectivity between the two catalysts, a heterogeneous kinetic model for MCH conversion accounting for adsorption and intrinsic catalytic reaction phenomena is considered. It is proven that there are significant variations between catalysts with respect to intrinsic kinetic constants associated with hydrogen transfer, ring-opening/cracking and transalkylation reactions, showing the importance of zeolite properties, particularly crystallite size, during the catalytic conversion of naphthenes. (C) 2012 Elsevier Ltd. All rights reserved.