In search for the optimal water treatment of a sulfate-containing water for drinking purposes from the location Prud (Croatia) nanofiltration experiments were undertaken to get data needed for the membrane treatment process considerations. The results of the nanofiltration experiments have shown very high retentions of sulfates by the examined nanofiltration membranes. The membranes' porosity characterization showed bimodal and trimodal pore size distributions (PSDs) with a large fraction of medium-size pores of approximately 0.85 nm in the skin of both examined NF membrane types. The effective number of pores in two types of the NF membranes differed by an order of magnitude. Although the retention of inorganic salts was primarily affected by the membranes' fixed charge, the sieving effect is also important especially for the retention of salts with the common anion. (C) 2004 Elsevier B.V. All rights reserved.