Intraparticle diffusion was investigated over well-defined and uniform pore-structure (WDUPS) model catalysts under reaction conditions for hydrodemetallization of heavy oil. Based on the intrinsic reaction rate constant and the apparent reaction rate constant, the intraparticle effective diffusivity was calculated. The intrinsic diffusivity and the bulk diffusivity under reaction conditions were obtained from the measured intraparticle effective diffusivity, and the porosity and tortuosity of the catalyst. The intrinsic diffusivity obtained using the WDUPS model catalyst was greatly different from that obtained using the conventional catalysts. Owing to its well-defined and uniform pore-structure and therefore definite porosity and tortuosity, the WDUPS model catalyst is more advantageous in intraparticle diffusion study than conventional catalysts, furthermore, the intrinsic diffusivity and the bulk diffusivity obtained in this research work are more reliable than those obtained before by using conventional catalysts with vague tortuosity and wide range of pore-size distribution. (C) 2013 Elsevier B.V. All rights reserved.