Reverse electrodialysis (RED) is a technology producing renewable energy from the mixing of river and seawater. In natural salinity gradients, multivalent ions are present, which lead to a reduced RED power output. Transport of multivalent ions against the concentration gradient and their trapping inside the membranes leads to a lower driving force and increased membrane resistance. The present work focuses on the effect of sulfate ions on anion exchange membranes in RED. A monovalent ion selective membrane ability to retain a higher open circuit voltage is offset by the higher resistance in the presence of sulfate, leading to losses in normalized power outputs (−25%) comparable to a standard grade membrane. Longer term experiments revealed that membrane resistance increases over time. This study highlights the need to address uphill transport, resistance increase, and decreased permselectivity of anion exchange membranes in presence of multivalent ions.