The diffusion and deactivation during methanol conversion over SAPO 34 were studied on differently sized crystals using an oscillating microbalance reactor. The reactive diffusivity of 1.1-10(-8) m(2)/s measured from methanol conversion on differently sized crystals was comparable with the methanol steady-state diffusivity of 3.3 x 10(-9) m(2)/s as measured by the uptake method. A percolation model based on a realistic three-dimensional pore network and taking into account the site coverage effect and the correlated percolation phenomenon, was developed to describe the diffusion and deactivation due to coke deposition. Simulated changes in relative effective diffusivity with the coke content or the fraction of occupied sites in the network were found to agree well with experimental results. The diffusivity decreased significantly with increasing coke content in the cages. Water adsorption was found to be a simple and reliable method to measure the pore volume changes with the coke content, relating the coke content with the fraction of occupied sites in the network.