The supermolecular structure of cellulose acetate membrane casting solutions was determined by measuring their rheological behavior. The method was applied to both clear and turbid casting solutions. The resulting data on activation energy of viscous flow are shown to depend on the cellulose acetate content of the casting solution and on the nature of solvent used. The separation properties of the prepared asymmetric membranes determined by reverse osmosis testing indicate a good correlation with the data obtained by the rheological measurements of the casting solutions. The latter can, therefore, be used as a practical tool for the investigation of early stages in the asymmetric membrane formation process.