The complexity of a heavy gas oil feedstock and the multitude of reaction pathways have limited previous attempts to model fluid catalytic cracking (FCC). The demand for more detailed kinetic information motivates the use of pure components to first elucidate the dominant pathways and mechanisms and then determine the associated rate parameters, including adsorption constants and heats of adsorption. The aim of the present work is to evaluate adsorption constants and heats of adsorption, under FCC relevant reaction conditions. The experiments are carried out in a novel CREC Riser Simulator (batch reactor unit) using USY zeolite catalysts with different crystallite sizes (0.4 and 0.9 microns). This study confirms a special feature of the CREC Riser Simulator, as a valuable tool for the study of adsorption phenomena. Adsorption constants and heats of adsorption are evaluated for benzene, toluene, xylene and trimethylbenzene, at initial reaction conditions. Catalytic conversion experiments for 1,2,4-trimethylbenzene help to demonstrate the consistency of the determined adsorption parameters at various temperatures and reaction times. In addition, adsorption constants and heats of adsorption are found to be constant throughout the reaction time and the formation of coke does not hinder the adsorption of 1,2,4-TMB, although it significantly affects the reactivity of this model compound.