In vacuum salt production sulfate is an important impurity, but it is also used to remove other cationic impurities from the raw brine. Removal of excess sulfate is currently done by purging salt crystallizer mother liquor from the brine plant, or crystallizing sodium sulfate through evaporative or cooling crystallization. The use of nanofiltration for the production of brines supersaturated in sodium sulfate is a technically feasible and attractive alternative. Crystallization of sodium sulfate in the membrane modules can be avoided by the presence of a primary nucleation inhibitor in the feed to the nanofiltration unit. Sodium sulfate crystallization can be induced in a separate crystallizer outside of the membrane unit by addition of crystal seeds. NF270 is a suitable membrane for this application and is preferred over Desal DK, Desal DL, TS-80 and MPF-34. Chloride and bromide retentions are negative and they are a function of the difference in sulfate concentration between concentrate and permeate. Carbonate retentions are linearly dependent on sulfate retentions. Calcium retention for NF270 is in excess of 95% and proven to be stable during 1200 h of continuous on-site pilot plant operation. The same applies for the potassium retention albeit at a lower level of 10%.