Nanofiltration membranes of the ERN, NF90, and SR90 brands have been studied in use for the treatment of model solutions simulating seawater with the varying salt content. It has been shown that the overall selectivity and efficiency for all types of the membranes increase with increasing pressure; the highest selectivity for anions and cations increases in the order ERN > NF90 > SR90, whereas the membrane efficiency grows as NF90 > SR90 > ERN. The measurement results also allow for the conclusion that the selectivity is slightly dependent on the permeate withdrawal rate and the salinity, the total hardness concentration is reduced by 90-95%, and the total dissolved salts (TDS) is reduced by 30-40%. With a TDS of 35 g/L, the operating pressure does not exceed 20 atm, which is more than twice lower than that for reverse osmosis membranes. Thus, these membranes can be used with a high efficiency for softening seawater and brackish water with various salt contents.