An improved explanation of the mechanism of the porous membrane formation with asymmetric structure has been proposed. It emerged from the correlation of the results obtained by the light-scattering measurements of the ternary membrane casting solutions and by the reverse osmosis and ultrafiltration testing of the asymmetric membranes prepared from these solutions. The results of anisotropic light-scattering measurements indicated the extent of supermolecular structurization of the casting solution which affected the porous structure formation in the membrane surface region. The variation of the casting solution composition and, particularly, the role of the CA/F ratio, the formamide content, and their influence on the asymmetric membrane properties were investigated and explained using the improved concept of the membrane-making mechanism.