Abstract Reverse osmosis experiments have been carried out using very weakly negatively charged cellulose acetate membranes and membranes made from sulfonated polysulfone which have cation—exchange properties. Experimental solutions consisted of hydrophobic charged surfactants and inorganic electrolytes. The rejection of the surfactants decreases markedly as the concentration of electrolytes increases and reaches negative values at low pressures. The volume flow in the presence of the surfactants also decreases. A membrane polymer/substrate interaction, e.g. adsorption, is proposed to account for this effect which is more pronounced with the ion—exchange membranes. However, at c.m.c. levels in the solution, the surfactant rejection increases and it appears to be related to a change in the activity.