Polydimethylsiloxane (PDMS)/polystyrene (PSt) interpenetrating polymer network (IPN) membranes were prepared by the bulk copolymerization of styrene and divinylbenzene in the PDMS networks. The interpenetration of PDMS and PSt resulted in the improvement of mechanical properties of PDMS. Transmission electron microscope (TEM) observation demonstrated that the PDMS/PSt IPN membranes have microphase-separated structures consisting of a continuous PDMS phase and a discontinuous PSt phase. When an aqueous ethanol solution was permeated through the PDMS/PSt IPN membranes by pervaporation, the PDMS/PSt IPN membranes exhibited ethanol permselectivity, regardless of the PDMS content. The effects of their microphase-separated structures on the permeability and selectivity for aqueous ethanol solutions are discussed experimentally and theoretically.