The invention concerns porous fibers and membranes, methods for their preparation and for their use. The fibers are characterized by a smooth porous surface and an apparent density of between about 10 and 90% of the true density of the polymeric starting material employed. The process involves the formation of a homogeneous mixture of at least two components, one of which is a meltable polymer and another liquid inert with respect to the polymer. The mixture formed must be of a binary type, in which there is a temperature range of complete miscibility and a range in which there is a miscibility gap. The mixture is extruded at a temperature above the separation temperature into a bath containing at least some of the inert liquid which is at a temperature below the separation temperature. Upon introduction of the mixture into the bath, the fiber structure of the product is fixed.