The effective diffusivity of the catalyst was experimentally measured in both octane-decane and polystyrene-chloroform systems for comparison with effects identified in the coal liquefaction model work. The diffusivity data were in good agreement with the theoretical solution and their reproducibility was satisfactory. In both experimental systems, the effective diffusivity was strongly dependent upon the ratio of diffusing species to catalyst pore size. When the steric exclusion-hydrodynamic drag theory was applied, the tortuosity significantly varied with the ratio of diffusing molecule to catalyst pore size. Using the diffusivity data of unimodal catalysts, an empirical equation relating the size ratio to the restrictive factor was derived to evaluate the diffusivity data of bimodal catalysts.