Pressure induced electrical potential across a composite polyamide/polysulfone reverse osmosis membrane and a microporous polysulfone membrane similar to the porous layer of the composite one (Delta E and Delta phi(st)(PS), respectively) were measured with NaCl and MgCl2 solutions at different feed concentrations. Because of the high salt rejection and the two-layered structure assumed for reverse osmosis membranes, Delta E values do not represent the streaming potential in the active layer since other electrical potentials (diffusion potential in the active layer, streaming potential in the porous sub-layer and boundary potential) are also included. These different terms were evaluated and the streaming potential for the active and porous layers of the composite membrane, Delta phi(st)(a) and Delta phi(st)(p), were estimated under the assumption of similar streaming potential coefficient g=Delta phi(st)/Delta P for the porous layer of the composite membrane and the microporous polysulfone membrane (g(PS)=g(P)). Differences between Delta E-m(i) values obtained under filtration (Delta E-m(F), different feed and product concentrations) and dialysis conditions (Delta E-m(D), equal feed and product concentrations), for similar volumen fluxes show the influence of the porous sub-layer. A comparison between Delta E-m(F)/Delta P and active layer streaming potential coefficient, g(a), is also made. Results show that the diffusion potential in the active layer is 10-15% of the total electrical potential difference across the composite membrane, while the influence of the porous layer is practically lower than 3%. The electrical potential in filtration measurements was calculated from dialysis results, once diffusion and streaming potential in the porous layer are known, and good agreement between experimental and calculated values was obtained. Salt concentration dependences for streaming potential coefficient in porous and active layers of the reverse osmosis membrane were studied, while for the microporous polysulfone membrane the zeta potential (zeta) was determined by the Hemholtz-Smoluchowski expression.