Abstract: A process for preparing ultra-filtration membranes and the product thereof, in which a selected cellulosic-ester polymer solution is cast into a thin film, at about ambient temperature, partially dried and immersed in a gelation bath at -20 .degree. to +30.degree. C. The gelation bath contains a sufficient percentage by volume of ethyl alcohol or other organic polar compound to ensure that a flux rate in excess of the initial minimum is achieved. In this way, ultra-filtration membranes having water flux rates selected from within the approximate range 14-10,000 gals./sq.ft./day at 100 psig and a molecular weight cut off above about 15,000, can be produced.