A theoretical model of dead-end microfiltration purification (MFP) of dilute suspensions is suggested. The model is based on a sieve mechanism of MFP and takes into account the probability of membrane pore blocking during MFP of dilute colloidal suspensions. An integro-differential equation (IDE) that includes both the membrane pore size and particle size distribution functions is deduced. According to the suggested model a similarity property is deduced, which allows one to predict the flux through the membrane as a function of time for any pressure and dilute concentration based on one experiment at a single pressure and concentration. For a narrow pore size distribution in which one pore diameter predominates (track-etched membranes), the IDE is solved analytically and the derived equation is in a good agreement with the measurements on four different track-etched membranes. A simple approximate solution of the IDE is deduced and that approximate solution as well as the similarity property of MFP process is in a good agreement with the measurements on a commercial Teflon microfiltration membranes. (C) 2002 Elsevier Science B.V. All rights reserved.