Abstract: Membranes which are useful in ultrafiltration processes are prepared by casting films from solutions of heteroaromatic polymers and subsequently quenching these films into solvents which cause gelation of the polymers. Examples of polymers employed in the preparation of microporous membranes are: poly-as-triazines, polyquinoxalines, poly (pyrazinoquinoxalines), polyquinolines and poly (anthrazolines). Membrane properties such as solvent flux, specific water content and pore size are readily controlled by proper choice of polymer concentration, solvents, and the addition of nonsolvents, salts and/or surfactants to the casting solutions.