Processes for using a thin film ultrafiltration membrane for the separation of low molecular weight solutes such as the separation of glucose from sucrose and higher molecular weight sugars and the separation of amino acids from oligopeptides and polypeptides. The membrane possesses desirable properties or characteristics, and may be prepared by casting a solution of a polyelectrolyte complex on a microporous support to form a thin film of polyelectrolyte complex on the surface of the support. The thickness of the membrane will range from about 600 to 3,000 A.