Energy is released when feed waters with different salinity mix. This energy can be captured in reverse electrodialysis (RED). This paper examines experimentally the effect of varying feed water concentrations on a RED system in terms of permselectivity of the membrane, energy efficiency, power density and electrical resistance. Salt concentrations ranging from 0.01 M to 5 M were used simultaneously in two stacks with identical specifications, providing an overview of potential applications. Results show a decrease of both permselectivity and energy efficiency with higher salt concentrations and higher gradients. Conversely, power density increases when higher gradients are used. The resistance contribution of concentration change in the bulk solution, spacers and the boundary layer is more significant for lower concentrations and gradients, while membrane resistance is dominant for high concentrations. Increasing temperature has a negative effect on permselectivity and energy efficiency, but is beneficial for power density. A power density of 6.7 W/m(2) is achieved using 0.01 M against 5 Mat 60 degrees C. The results suggest that there is no single way to improve the performance of a RED system for all concentrations. Improvements are therefore subject to the specific priorities of the application and the salt concentration levels used. Regarding ion exchange membranes, higher salinity gradients would benefit most from a higher fixed charge density to reduce co-ion transport, while lower salinity gradients benefit from a thicker membrane to decrease the osmotic flux. (C) 2013 Elsevier Ltd. All rights reserved.