Wheeler has summarized the work on internal diffusion for catalytic cracking of gas-oil. At 500C the rate data for fixed-bed operation, with relatively large (1/8in) catalyst particles and that for fluidized bed reactors (very small particle) are about the same. This suggests that the effectiveness factor for the large particles is high. Confirm this by estimating for the 1/8 in catalyst is the more pore radius is 30A, the particle diameter is 0.31 cm, and the pore volume is 0.35 cm3 /g catalyst. Molecular weight of oil is 120.At atmospheric pressure with 30 A pores the diffusion will be Knudsen type. The rate data, interpreted in terms of a first order rate equation, indicate that k1exp=0.25 cm3 /s.g cat. Assume that the parallel pore model with a tortuosity factor of 2 is applicable.