A chromatographic pulse injection technique has been used with a modified Wicke-Kallenbach diffusion cell to measure the diffusivity of He-N2 mixtures in silica-alumina, a commercial zeolitic cracking catalyst and chromia-alumina pelleted catalysts containing increasing amounts of coke. While the results confirmed that diffusivity decreases with increasing coke content, the variation in the measured diffusivity with temperature suggested that diffusion occurs in the transition region for all pellets. The pore size distribution analysis indicated that the coke has a considerable blocking effect on the mesopores for silica-alumina and chromia-alumina catalysts while for zeolites the large mesopores and some macropores are affected.