The effect of high levels of coke deposit on catalyst properties and mass transport through the pores space was studied using commercial catalysts, Pt-Al2O3 and Ni-MoAl2O3. Changes in porosity, pore-size distribution and internal surface area of several aged catalysts were compared to those of fresh catalyst. Effective diffusivity was also measured by pulse chromatography. For Pt-Al2O3, which shows a high initial porosity (0.93) and a relative high mean pore radius (300 Angstrom), catalyst properties remain unchanged while coke content increased. For the Ni-Mo-Al2O3 catalyst (with initial porosity of 0.53 and mean pore radius of 50 Angstrom), however with an increase of 15% in coke content, isolated void regions appear, and porosity, internal surface area and effective diffusivity decrease sharply. Pore and stochastic models were used to interpret the effective diffusivity experimentally measured. A structural parameter in each case, tortuosity factor in pore models, and coordination number in the percolation model were calculated. When coke produces significant changes in catalyst morphology, the tortuosity factor almost doubles its initial value for a coke content of 20%. On the contrary the percolation model seems to predict changes accurately in porosity, mass transport coefficient, and an isolated porosity growing from the coordination number matched for a fresh catalyst.