Reverse electrodialysis (RED) is a sustainable energy source that converts the free energy of mixing of two solutions with different salinity directly into electrical energy. High power outputs are predicted theoretically, but the practical power output and the efficiency of the process are still relatively low. Next to concentration polarization phenomena, especially the spacer shadow effect, which occurs due to the use of non-conductive spacers, has a high impact on the net power output. Non-conductive spacers block the ionic transport in the stack thus reducing the area effectively available for ionic transport. In this work we developed and applied ion conductive spacers for RED, which eliminates the spacer shadow effect and obviously improves the overall process performance. We combined theoretical calculations with direct current and alternating current experimental stack characterization methods to quantify this effect on stack resistance and power output in RED process under different hydrodynamic conditions. The use of ion conductive spacers led to a decrease in stack resistance and an increase in power density of a factor 3 compared to the use of non-conductive spacers made of PVC with the same open area and shape. This result increases the impact of RED as renewable energy source and brings it closer to economical viability and practical implementation. (C) 2009 Elsevier B.V. All rights reserved.