The fabrication of polymeric microsieves with tunable properties (pore size, shape or porosity) is described in this work. Perfectly structured freestanding membranes and accurate replicas of polyethersulfone (PES), copolymers of polyethersulfone and polyethylene oxide (PES-PEO), and blends of PES and hydrophilic additives were produced by phase separation micromolding (PSRM) using a microstructured mold. Phase separation occurred in two stages: vapor-induced phase separation (VIPs), where shrinkage and subsequent perforation of the polymer film took place, and liquid-induced phase separation (LIPS), where lateral shrinkage that facilitated the release of the polymer replica from the mold occurred. The dimensions of the perforations were tuned either by using molds with different pillar diameter or by thermal treatment of the polymer above its glass transition temperature. By the latter method, microsieves with initial pore sizes of about 5 or 2.5 mu m were reduced to 1.5 and 0.5 mu m, respectively, whereas perforations down to 1.2 mu m were achieved by tuning the dimensions of the mold features. (c) 2006 Elsevier B.V. All rights reserved.