The particle layer build-up during crossflow microfiltration (CMF) has been examined experimentally. To gain more information than only the time dependent filtrate flux. specially designed flat duct modules have been developed. These modules provide defined flow conditions and allow an easy removal of the particle layer covered membrane. The particle layer thickness and topography, the specific mass flow rate and the particle size distribution were recorded. The deposition process was examined in turbulent and in laminar flow as well as for particles having a negligible density difference compared to the liquid. These results led to a new process of wet classification, that takes advantage of the primary disadvantegeous effect of selective particle deposition. The aim of the new process is to win the particle layer as fine product. The process is subdivided in two steps. In the first step, the fines deposit on the membrane and then are discharged by back-flushing in the second step. A new pilot plant with tubular membranes shows the transferability of the flat duct experiments and proves the feasibility of the two process steps. The influence of the determining parameters flow velocity, filtrate flux and particle concentration has been studied. (C) 2002 Elsevier Science B.V. All rights reserved.