Both in electrodialysis and in reverse electrodialysis ionic shortcut currents through feed and drain channels cause a considerable loss in efficiency. Model calculations based on an equivalent electric system of a reverse electrodialysis stack reveal that the effect of these salt bridges could be reduced via a proper stack design. The critical parameters which are to be optimized are ¿/r and R/r, where ¿ is the lateral resistance along the spacers, R is the resistance of the feed and drain channels between two adjacent cells, and r is the internal resistance of a cell. Because these two parameters are dimensionless, different stacks can be easily compared. The model is validated with two experimental stacks differing in membrane type and spacer thickness, one with large ionic shortcut currents and one where this effect is less. The loss in efficiency decreased from 25 to 5% for a well-designed stack. The loss of efficiency in reverse electrodialysis and in electrodialysis can be reduced with the aid of the design parameters presented in this paper.