In principle, microsieve membranes have high fluxes, due to their extremely low flow resistance and their uniform pore size. However, it was found experimentally, that the design of the support structure, even though its flow resistance is negligible, had great effect on the flux and the evolution of pore blocking. This finding was quantified using computational fluid dynamics (CFD) simulations of the flow through the microsieve. From the CFD calculations, we could conclude that the design of a microsieve should necessarily encompass the design of the top layer and the support structure together. The first design of the microsieve only had 30% of the maximum possible flux. It was shown that the channel height between the pore field and the support structure should be at least 150 mu m for optimal use of the microsieve. (c) 2005 Elsevier B.V. All rights reserved.