A novel polymer membrane with nanosized pore structures has been prepared from the direct copolymerization of acrylonitrile (AN) with a polymerizable nonionic surfactant in water-in-oil (w/o) or bicontinuous microemulsions. This polymerizable surfactant is omega-methoxy poly(ethylene oxide)(40) undecyl-alpha-methacrylate macromonomer [CH3O-(CH2CH2O)(40)-(CH2)(11)-OCO(CH3)C=CH2, abbreviated: C-1-PEO-C-11-MA-40]. Besides PEO macromonomer, AN, and crosslinker ethyleneglycol dimethacrylate, the microemulsion system contained varying amount of water that formed w/o microemulsions having water droplet structures and bicontinuous microemulsions consisting of interconnected water channel. The polymerized membranes prepared in this study have pore radii ranging from 0.38 to 2.4 nm as evaluated by PEG filtration. The pore size appears to vary linearly with water content in precursor microemulsions. But a sharp change in the gradient of the linear relationship is observed around 25 wt % water content. Membranes made from bicontinuous (>25 wt % water) microemulsion polymerization have a larger and interconnected (open-cell) nanostructures. In contrast, much smaller closed-cell (disinterconnected) nanostructures were obtained from w/o (<25 wt % water) microemulsion polymerization and the membrane exhibited a permselectivity toward water in pervaporation separation of high ethanol (>50 wt %) aqueous solutions. The separation factor (alpha) for 95% ethanol aqueous solution by the membrane derived from the microemulsion containing 10 wt % water is about 20.