The possibility to obtain ultrafiltration polyacrylonitrile terpolymer membranes in the presence of formamide in dimethylformamide polymer solution was studied. Correlation between the changes in viscosity and rheological characteristics of the polymer solutions, on the one hand, and the equivalent pore radii, efficiency, and selectivity of membranes, on the other hand, were found. The concentration of formamide was varied from 0 to 4 mass %. The dynamic characteristics of the membranes were determined with regard to calibrators having different molecular weight.