Effective diffusivities of polyaromatic solutes in aliphatic solvents were determined from adsorption measurements a t ambient conditions with three hydrotreating catalysts of pore diameters 6.2-17.5 nm, Solutes studied were disklike (coronene, 9-phenylcarbazole, tetraphenylporphyrin, and tetra(4-biphenyly1)porphyrin) and rodlike (naphthalene, anthracene, benzanthracene, and dibenzanthracene) polyaromatics of critical dimensions ranging from 0.74 to 2.18 nm. A modified Wilke-Chang-type correlation was developed to predict bulk diffusivities of the polyaromatic solutes in aliphatic solvents. The effective diffusivities and bulk diffusivities were then used t o determine restrictive diffusion correlations. The structure of saturated hydrocarbon solvents (n-heptane, isooctane, and cyclohexane) was found to have little effect on restrictive diffusion. The critical solute dimension gave the best correlation for restrictive diffusion of nonspherical molecules in catalyst pores. The resultant correlation, based on the ratio of solute critical dimension to average pore dimension, was found to be in close agreement with hydrodynamic theory.