Reverse electrodialysis (RED) is an emerging technology that can generate electricity from the mixing of two water streams (i.e., the concentrated and the diluted streams) with salinity gradient. In RED, the higher salinity gradient between water streams yields the higher power production. Therefore, water sources containing a high concentration of salts such as reverse osmosis brine, hypersaline lakes, and produced water from hydraulic fracturing could be considered as feed streams for enhancing energy production in RED. However, these water sources contain not only NaCl but also various multivalent ions, which are likely to increase electrical resistance of ion exchange membranes (IEMs) and potentially decrease power generation. In this study, we investigated the effects of divalent cations in the concentrated stream, including magnesium, calcium, and barium ions on electrical resistance of IEMs in static mode. The electrical resistance of IEMs in static mode was found to be correlated to power production in a bench-scale RED process during continuous operation. As a result, it was found that divalent cation with the smaller hydrated radius showed the higher electrical resistance in the static mode and the increased electrical resistance of cation exchange membrane (CEM) resulted in power reduction during the continuous operation of the bench-scale RED process.