The prospects and potential of Reverse Electrodialysis (RED) for energy harvesting from natural streams with salinity gradient demand more in-depth studies to understand and overcome the limitations posed by divalent ions. Power performance is greatly influenced by the ionic resistance displayed by the alternating cation and anion exchange membranes (CEMs and AEMs, respectively) housed in RED stacks, which in turn is determined by the type and concentration of ions and counter-ions in the water streams. The effects of divalent ions on power output have been experimentally approached in several works by using real or synthetic water. However, the development of comprehensive models including the effect of divalent ions on membrane resistance and power performance under different scenarios is still very scarce. Thus, this work investigates experimentally the effect of ion species on membrane resistance, providing for the first time mathematical correlations useful to predict power performance in RED stacks under a wide range of compositions of salinity gradient solutions. To this end, electrochemical impedance spectroscopy (EIS) measurements have been performed for CEM and AEM commercial membranes in contact with different concentration of NaCl solutions and including different mixtures of divalent ions (Ca2+, Mg2+, SO42−). These correlations have been implemented in a previously developed model to determine power outputs as function of ion mixture compositions. Scenarios of general interest for RED practical implementation have been addressed; specifically, solutions with a composition representative of seawater or high salinity brines have been studied as high concentration solutions (HCS) and, on the other hand, typical concentrations of wastewater treatment plant effluents, river water or brackish water from desalination plants were used as low concentration solutions (LCS).