The volume flow through a Nafion membrane originated by the simultaneous action of an electric potential difference and a pressure difference has been measured using aqueous KCl solutions under different experimental conditions. The behavior has been analyzed when both gradients act in the same and in the opposite sense. The results indicate that the simultaneous action of the pressure and potential differences originates a total flow different from the sum of the electroosmotic and permeation flows due to each force acting separately. The application of irreversible thermodynamics, which includes second-order terms, allowed the determination of the phenomenological coefficients. Moreover, from these values, the equivalent pore radius was estimated on the assumption that the membrane is a porous medium filled with an internal solution. (C) 2004 Elsevier Inc. All rights reserved.