It was studied the effect of using ethanol as co-solvent in the CO2 foaming process of polymeric membranes for microfiltration applications. Flat membranes of partially miscible blends of PC/PMMA were prepared through a one-step foaming method. The interaction and miscibility between the phases in the polymer blend was characterized by DSC, whilst the morphologies of the flat membranes were analyzed by scanning electron microscopy (SEM). The permeability of nitrogen and water through the membranes were evaluated by appropriated techniques. This study showed that foaming homopolymers solely with CO2 generates closed cell foams with a dense skin layer whereas foaming the PC/PMMA blend solely with CO2 still formed closed pore walls with dense skin layer. On the other hand, the polymer blend foamed with the ethanol/CO2 mixture showed a very interesting morphology where the ethanol/CO2 mixture led not only to the openness of the pore walls in the polymer blend, but also inhibited the formation of the dense skin layer. Bubble point tests were performed for the PC/PMMA sample foamed with ethanol/CO2 mixture and the results displayed a pore size distribution suitable for microfiltration applications.