The alkaline hydrolysis of asymmetric cellulose acetate membranes was investigated. The changes of cellulose triacetate and cellulose diacetate membrane performances caused by hydrolysis of the membrane material were measured, and the data were analyzed in terms of pore size distributions and effective number of pores in the upper membrane layer before and after hydrolysis. The resulting membranes rejection factors decrease of 10-25%, and a noticeable permeation rates increase were caused by closing of a part of the smallest "network" membrane pores and by increasing the size of the other pores.