Particle capture at membrane surfaces in cross-flow microfiltration is studied both experimentally and computationally to better understand permeability reduction as function of membrane pore sizes and particle sizes and concentrations. Microfiltration experiments for various bentonite suspension concentrations are conducted in commercial polysulphone membranes of nominal pore sizes 2 and 0.2 mum. Permeability reduction is continuously evaluated. Scanning electron microscopy (SEM) and image analysis are used to determine membrane pore size and particle size distributions. For simulation purposes. the membrane pore space is represented by a bundle of nonintersecting tubes. Pore segments in the model membranes have circular cross-sections. random locations and sizes distributed according to distributions determined experimentally. Monte Carlo simulations of cross-flow microfiltration of a well-characterized particle suspension on well-defined model membranes are presented and permeability reduction calculated. Particle capture and size exclusion at pore segments are considered the dominant mechanisms of membrane fouling, In the simulations a matching size criterion between pore size and particle size is used to define pore blocking. Simulation results accord with those obtained experimentally, Permeability decreases more rapidly at higher particle concentrations. For all particle concentrations, permeability reduction of the thinner pore membrane is more intense. Membrane permeability scales linearly with porosity in both cases of membranes. This result is discussed in the light of simple theoretical arguments.