A mathematical model is considered to describe the importance of the adsorption of hydrocarbon species in FCC risers. Results show that both the hydrodynamics and the reaction kinetics are strongly influenced by adsorption phenomena. Vapor exit velocities based on pseudo-homogeneous models (no adsorption included) are significantly larger, about 2 times, than gas velocities calculated with the heterogeneous model (adsorption included). The proposed heterogeneous reaction model also predicts increases in catalyst and vapor residence times and consequently higher gas oil conversions.