Two typical FCC catalysts with similar acidities and structural properties and different crystallite sizes, CAT-SC (0.4 mum) and CAT-LC (0.9 mum), are studied in a novel CREC riser simulator using 1,3,5-triisopropylbenzene. Catalytic and thermal runs allow for the development of a heterogeneous kinetic model and the assessment of intrinsic kinetic constants and adsorption and diffusional parameters. Analysis of 1,3,5-triisopropylbenzene conversions and product distribution helps establish the influence of intracrystallite diffusion as the controlling step at lower reaction temperatures (350-450 degreesC). Moreover, product distribution and selectivity differences between the CAT-SC and CAT-LC catalysts provide evidence of cracking reactions affected by hindered diffusion of the reactant and products.