In this investigation organic fouling of ED membranes was studied with and without applied voltage. Fouling without the application of voltage, i.e., adsorption, was observed as an increase in the membrane resistance, and fouling with the application of voltage, i.e., conventional ED, was observed as an increase in the voltage drop across the membrane. Experiments were performed with three different carboxylic acids (propanoic, octanoic and decanoic acid) and an alkaline bleach plant filtrate from a sulphate pulp mill. An anion-selective (Selemion AMV) and a cation-selective (Selemion CMV) membrane were used in the investigation. A significant difference was observed between the fouling of the two membranes. The membrane resistance and voltage drop across the cation-selective membrane did not increase in any of the experiments, i.e., the cation-selective membrane was not fouled. The anion-selective membrane, on the other hand, was fouled by all solutes except sodium propanoate.