In this investigation, the influence of various kinds of organic solutes on the fouling of an anion and a cation selective ED membrane was studied. Fouling by adsorption of organic matter onto the membrane was measured as an increase in the membrane resistance with time. Experiments were performed with a fatty acid (octanoic acid), two anionic surfactants (sodium octanoate and sodium dodecylbenzene sulphonate) and an alkaline bleach plant filtrate from a sulphate pulp mill. A marked difference was observed between the increase in the membrane resistance of the anion selective membrane and that of the cation selective membrane. The cation selective membrane was slightly fouled by the bleach plant filtrate, but was only marginally affected by the other organic solutes. The anion selective membrane, on the other hand, was markedly fouled by all solutes.