The evolution of the effective diffusivity of a pseudo-continuum model of a ZSM5 particle in which progressive blockage occurs is calculated from a network model using bond-and site-percolation theory and the effective medium approximation (EMA). Reduced clusters of varying complexity are evaluated with respect to their capability of accurately predicting the relationship between the evolution of the effective diffusivity with the blockage. A combination of EMA and scaling leads to an excellent agreement with the results of Monte Carlo simulation on a 20 x 20 x 20 lattice. The retained relationship is used in the simulation of reaction and diffusion in a ZSM5 particle of a fixed bed reactor. The catalyst is subject to deactivation by coke formation leading to site coverage and pore blockage.