Transport characteristics of nanofiltration membranes OPMN-K, OPMN-KM3, OPNTN-P, and OPMN-KN (produced by "Polimersintez", Russia) were studied in the process of separation of low concentration solutions of salts NaCl, Na2SO4, CaCl2. Operation areas of the membranes were determined for each salt solution, where the selectivity-volume flow relationship reached its saturation. The data were obtained showing a predominant role of the charge, which determines selectivity of membranes under conditions of separation. We have analyzed a relationship between experimental results and data on the composition of polymers that are used in synthesizing the membranes studied. On the basis of the experimental results an attempt was made to predict the selectivity of membranes in separation of low concentration solutions of salts.