An experimental study of the relationships between coke content, catalyst activity, and intraparticle diffusivity is reported for cumene cracking on a commercial H-mordenite. Coking conditions were varied from 230 to 350 °C at space velocities from 0.20 to 0.65 wt/hr/wt and time on stream up to 6 hr. Effective diffusivities of the catalysts were found to decrease by over a factor of 2 during the coking process and are nonlinearly related to the coke level, independent of the temperature of coking. Under conditions of strong diffusion limitation, this change in diffusivity alters the catalytic effectiveness in proportion to the square root of the diffusivity ratio, ( D eff D 0 eff ) 1 2 .