This study examines the roles of intracrystallite diffusion and reaction phenomena during the catalytic cracking of vacuum gas oil. Catalytic cracking experiments on FCC-type catalysts were performed in a fluidized bench-scale CREC riser simulator. This reactor was operated under close-to-industrial FCC conditions in terms of temperature, reaction time, partial pressures of reactant and products, and catalyst-to-oil ratio. The activity and selectivity of two USY zeolite catalysts, with very similar properties but varying zeolite crystallite sizes. were determined. A five-lump kinetic model describing the catalytic cracking of gas oil to light cycle oil (gasoline, light gases, and coke and accounting for diffusional constraints experienced by hydrocarbons while evolving in the zeolite pore network was considered. The results show that the catalyst with the smaller crystallites provided higher activity and selectivity toward desirable intermediate products (gasoline with low aromatics) and lower selectivity for terminal products (coke), indicating that diffusion plays a significant role in catalytic cracking. Diffusivity and kinetic parameters, including modified Thiele modulus and effectiveness factor, were established to determine the effects of crystallite size and temperature oil the operating regime of the catalyst. It was found that, in the 510-530 degrees C range, the overall cracking rate is controlled by the highly temperature-sensitive intracrystalline gas oil transport, whereas in the 550-570 degrees C range, the overall cracking rate is dominated by a mildly temperature-sensitive intrinsic cracking rate.