In order to elucidate the rejection properties of FT30 membranes, reverse osmosis experiments were carried out with varying ion concentration (1-40 mol/m(3)), ion composition, transmembrane pressure drop (1-3 MPa) and pH (4-10). As model solution for each experiment demineralized water was used containing known amounts of salts such as NaCl, NaNO3, KCl, KNO3, Na2SO4 and CaCl2 either single or in combination. Some experiments were made with solutions containing compounds that cause fouling of the membrane such as Fe(OH)3 or humic substances. Specific rejection and overall salt rejection were determined by measuring the ion concentrations and electrical conductivity in samples of the permeate and concentrate. The results showed an increase of rejection with transmembrane pressure drop and a decrease in rejection with increasing concentration, whereby the latter effect was more distinct in mixed salt solutions than in single salt solutions. Rejection of anions and cations increased with pH as the membrane charge became more negative. Multivalent ions were better rejected than univalent ions. Nitrate was better rejected as single solute than in combination with other salts in solution. If fouling layers were formed on the membrane surface, the rejection properties changed. Fouling layers made of iron hydroxide caused a marginal decrease in salt rejection whereas fouling layers made of humic substances caused an improvement of salt rejection under the conditions used.