The effect of the ratio of reactant molecular diameter to catalyst pore diameter on the restrictive diffusion of reactants in catalysts under catalytic hydroprocessing conditions was studied. Hydrogenation of four nitrogen-containing heterocyclic model compounds dissolved in decalin was carried out in a batch-mode operation at 623 K and 5.27 MPa of hydrogen partial pressure over three Ni-Mo/alumina catalysts encompassing a range of molecular to pore sizes of 0.03 to 0.35. An empirical correlation for restrictive diffusion under realistic processing conditions was developed. The correlation shows that the effective diffusivity is strongly influenced by the ratio of the reactant molecular size to the average micropore size, taking into account changes in pore sizes due to coke deposition. The restrictive effect under reaction conditions was found to be larger than the theoretical prediction for nonreactive conditions.