The volume flow of methanol-water potassium chloride solutions through a Nation membrane originated by the simultaneous action of electric potential and pressure gradients has been measured at different percentage, of methanol. Measurement were conducted when both gradients act in the same and in the opposite directions under different experimental conditions, The result, indicate that the simultaneous action of the pressure and potential differences originates a total flow different from the sum of the individual electroosmotic and permeation flows due to each force acting separately. The application of the irreversible thermodynamics theory. which includes second-order terms, allowed the study of the influence of the composition of the solutions on the determination of the different phenomenological Coefficients. (c) 2005 Elsevier Inc. All rights reserved.