The influences of membrane type and composition on fouling during crossflow microfiltration of particulate suspensions are discussed with relation to data obtained from sequences of computer controlled experiments. A number of commercially available polymeric membranes were identified, characterised and challenged with particulate streams of known size, shape and surface charge at a range of well defined, constant process conditions. The flux declines observed during microfiltration are related to the known characteristics of the particle stream and the filtering membrane septum. The fine particles in the feed suspension are shown to control the rate of filtration, and render the rate insensitive to membrane pore size or size distribution. The greatest rate of filtration is obtained with membranes whose pore sizes are smaller than the finest particles in the feed stream. Effects of membrane hydrophilicity/phobicity are short lived, and surface charge effects are secondary.