A kinetic model is developed to describe the product distribution of gas oil cracking in a novel riser simulator. Typical estimated diffusivity of gas oil molecules in 0.1 mu m Y-zeolite crystals leads to effectiveness factors of 0.8-1 while for 1.0 mu m crystals leads to effectivenss factors of 0.1-0.25. The proposed model predicts that, for medium/low gas oil conversions, the olefin lump is found to be the most abundant product lump followed by paraffins, aromatics, and naphthenes. The model also shows that for high gas oil conversions the paraffin lump increases at the expense of the olefin lump. It is found that 0.1 mu m zeolites produce, at moderate/high gas oil conversions, more olefins and less aromatics than 1 mu m zeolites. Selectivity differences are explained on the basis of intracrystal diffusivity effects on both cracking and hydrogen-transfer reactions.