Diffusion and reaction in composite catalyst particles were calculated using the dusty-gas-model, Monte Carlo and Molecular dynamics calculations for the determination of multicomponent diffusivities of molecules participating in the methanol-to-olefins synthesis. A three-dimensional model of zeolite structures is presented. Without a fitting parameter, the diffusivities calculated in silicalite give a good approximation to the experimental data. Diffusion and reaction of the MTO synthesis is described with a model employing about 4000 particles.