Ion transfer in the pores of semipermeable cellulose diacetate membranes during the ultrafiltration of aqueous solutions containing high-molecular-weight compounds (serum proteins) and mineral substances including Cl-, K+, Ca2+, and Na+ ions is considered. It was found experimentally that the concentrations of not only proteins but also the above ions changed in the course of membrane filtration. A model was proposed for ion transfer in the capillary pores of ultrafiltration membranes, and the numerical values of Ca+ and Cl- transfer parameters in the test systems were obtained.