The alkylation of benzene with light hydrocarbons over an acidic catalyst is an important industrial process. Metal halides and solid phosphoric acid have been the most commonly used acid catalysts for this purpose, while zeolite catalysts are excellent, environmentally clean alternatives. Although Liquid-phase alkylation with certain synthetic zeolites has been established commercially (such as in catalytic distillation), little has been published on the fundamental parameters governing the reaction kinetics. This paper examines the effect of external (liquid-solid) and internal (pore diffusion) mass transfer on the main reaction of benzene alkylation with ethylene and propylene using various zeolite catalysts. The results and its implications on the effective catalyst utilization will be presented. This constitutes one of the series of basic studies in an effort to optimize aromatic alkylation reactor systems such as catalytic distillation.