The inherent challenge of the forward osmosis (FO) process is the severity of both external (ECP) and internal concentration polarization (ICP), which significantly reduces the water flux across the highly selective membrane. In this study, the impacts of concentration polarization on flux behavior were investigated. A modified-film model developed using the boundary layer concept described the ECP layer much better than previously used models. By including the diffusion coefficient into the derivative of the governing convective-diffusion equations, the predicted water flux due to ICP was in excellent agreement with experimental flux data. This was attributed to the usage of a better solute resistivity constant within the porous support layer, K*, which is independent of the diffusivity coefficient. Laboratory experiments were carried out to account for both ECP and ICP and the associated water fluxes were verified with the improved models. Previous models overestimated the water flux by as much as 15% of the experimental flux and the modified models showed significant improvements in flux prediction for the FO process, particularly at higher draw solution concentration. A better understanding of the effects of concentration polarization achieved from this study could allow us to further modify the FO membrane structure to improve water flux. (C) 2008 Elsevier B.V. All rights reserved.