Isothermal matter transport due to gradients of chemical potential was studied with different electrolytic solutions and an hydrophobic porous membrane. Countercurrent fluxes of solute and solution in bulk were observed in each case. impeding volume flux, transmembrane pressures are developed proportional to concentration gradients and much smaller than Van’t Hoff osmotic pressures, indicating a poor membrane reflectivity. Starting from our experimental observations we propose a simple model of an idealized membrane, whose pores interact only hydrodynamically with bulk flow, while countercurrent diffusion of solvent and solute occurs in each channel along the respective gradients of chemical potential. This model is shown to work in every respect just as real-life membranes.