In this paper, nanofiltration (NF) combined with continuous variable volume diafiltration (CVVD) of cottage cheese whey (also known as acid whey) was investigated to simultaneously concentrate and partially demineralize it. The experiments were carried out in a laboratory NF equipment using a flat sheet membrane with the effective area of 0.046 m(2) and 96% sucrose rejection. Fat, protein, lactose and total solid content of the samples were analyzed by Infrared Milk Analyzer Bently 150. Ion composition of the samples was determined by ICP-MS instrument. The concentrations were carried out at different values of the ratio of the volume of dialyzation water and the volume of permeate (alpha). The degree of demineralization of monovalent ions reached 70% and 90% at 2.5 volume concentration ratio, and at diafiltration and permeate volume ratios of 0.5 and 0.75, respectively. The amount of multivalent ions increased continuously during the NF combined with CVVD of acid whey. The rejection of the valuable compounds of whey such as protein and lactose was over 90%.