A membrane of defined pore structure and controlled pore diameter and a method of preparing the membrane which comprises dispersing in a water soluble polymer solution (A) an organic polymer solution (B), the water-soluble polymer solution (A) being a non-solvent for the polymer solution (B), to form micro-spherical droplets of the polymer solution (A) which are enveloped by a solid phase consisting of a coagulated or cross-linked polymer solution (B); evenly casting the dispersion on a flat surface; and evaporating the solvent for the polymer solution (B) to form a microporous membrane comprised of the polymer (B), the porosity, pore size, and void volume of the microporous membrane being a function of the polymer (A) concentration, the microsphere dimensions and the temperature and evaporation rate. The water-soluble polymer (A) is selected from polyethyleneimine, gelatin, polyvinyl-pyrrolidone, polyvinyl pyridinium halide, polyacrylic acid, polyhydroxyethylmethacrylate (HEMA), dextran, or polyvinyl acetate. The continuous phase polymer (B) is selected from a brominated polyphenylene oxide, polyphenylene oxide, polyacetic acid, polystyrene, polyglycol acrylate (PGA), polymethylmethacrylate, cellulose acetate, polyvinyl acetate, polycarbonate and polyvinylidene fluoride.