AbstractHydrogenation is a complex 3‐phase reaction, and the end result depends largely on transport and chemical resistances. Control of these factors is of critical importance to a predictable end product and this control is possible if a fundamental understanding of the process is achieved. The basic transport steps involved to bring the hydrogen and oil together in the presence of catalyst and the effect of different process variables on the course of the chemical reaction will be the major subjects of discussion in this presentation.