We present a method for the simulation of catalyst fouling at the particle and reactor levels. At the particle level, diffusion and reaction are described by a continuum model. The accessible pore surface area and the effective diffusivity are obtained from percolation theory. Both these quantities depend on location within the particle, and vary with time. At the reactor level, a mixing-cell model is employed. We demonstrate the method by applying it to the hydrodemetallation of oil. Hydrodemetallation performance depends strongly on the pore structure of the catalyst, and there is a high degree of interplay between events occurring at the particle and reactor levels.