Microporous flat film membranes from immiscible polypropylene (PP)/polystyrene (PS) blends with and without a compatibilizing copolymer were produced via melt processing and post-extrusion drawing. The blends were first compounded in a corotating twin-screw extruder and subsequently extruded through a sheet die to obtain the precursor films. These were uniaxially drawn (100-500%) with respect to the original dimensions to induce a microporous structure and then post-treated at elevated temperatures to stabilize the porous structure, which consisted of uniform microcracks in the order of a few nanometers in width. The effects of composition and process parameters on porosity and solvent (methanol) permeability of the prepared membranes are presented. Comparison of the data with those of commercial membranes prepared by phase-inversion processes suggests significant differences in pore size distribution as well as overall surface porosity.