Previous work on the simulation of diffusion in porous materials and, in particular, catalyst support pellets has, in general, made use of abstract mathematical models to represent the structure of those materials. In this work; a magnetic resonance image of the spatial distribution of voidage within a thin slice through a porous catalyst support pellet is used as the basis for a potentially more realistic representation of the porous structure. Simulations of diffusion are performed using a simulation lattice derived from the image and the resulting predictions of tortuosity are compared with experimentally determined values obtained using PGSE NMR and magnetic resonance imaging techniques. This approach is applied to the study of five different types of catalyst support pellet,