The influence of the membrane characteristics on the water flux in the osmotic evaporation process, is evaluated. Due to the favourable hydrodynamic conditions used, concentration polarization was negligible. The effect of the membrane pore diameter, is evaluated, using symmetric membranes with the same characteristics except the pore size. The water vapour transport in the membrane pores is described by the Dusty-gas model (DGM), and the membrane tortuosity value is obtained. For the membranes used (0.10 mum less than or equal to d(p) less than or equal to 0.45 mum), operating at ambient temperature and pressure, both Knudsen and molecular diffusion are present. Their relative contribution to the membrane resistance depends on the membrane pore size: molecular diffusion increases and Knudsen diffusion decreases as the pore diameter becomes larger. Higher fluxes are obtained using a composite membrane, which is constituted by a thin active layer held in a support. The relative resistance of the support to mass transfer is determined experimentally. The magnitude of the temperature polarization is small for the studied system. In fact, the maximum driving force reduction due to this phenomena is 10%, for the composite membrane. (C) 2003 Elsevier B.V. All fights reserved.