Acrylonitrile copolymer membranes were subjected to partial hydrolysis in order to improve their dialysis characteristics. The partial hydrolysis time and temperature, as well as the concentration of the hydrolyzing agent, were the variable parameters. The changes in the chemical and physical characteristics of the membrane were proved by infrared spectroscopy, elemental analysis, differential scanning calorimetry, and Mercury intrusion porometry. After each degree of modification, the permeability of the membrane with respect to a solution of vitamin B12 was determined under static conditions and at temperature of 298 K. The improvement in the permeability at a definite degree of hydrolysis is explained by the conversion of nitrile groups to amide and carboxyl groups, as well as increase of the volume of the pores with average radius. The efficiency of the modified PAN membranes for concentration of biological liquids (diluted serum) was studied.