Reverse electrodialysis (RED) generates electricity through the entropy increase from the mixing of sea and river water. Two important RED process parameters were investigated: power density (in Watts per square meter membrane) and the thermodynamic efficiency. Beside this, we quantified losses in a RED stack experimentally: co-ion transport, osmosis and internal loss as a function of current density and related those to the energy loss due to mixing. Six different commercial available membrane pairs – Qianqiu Heterogeneous Ion Exchange Membrane (AEM and CEM), Qianqiu Homogeneous Ion Exchange Membrane (AEM and CEM), Fumasep (FKD and FAD), Selemion (AMV and CMV), Neosepta (ACS and CMS) and Neosepta (AMX and CMX) – were compared. It was found that at maximal power density, the thermodynamic efficiency was between 14% and 35%, where 50% is the maximum theoretical value at maximal power density. The highest score (35%) is achieved with the Neosepta (ACS-CMS) pair. The power density of the different membrane pairs varied from 0.5 to 1.2 W/m2. The maximal value was found for Fumasep (FAD and FDK) and Selemion (AMV and CMV) pair. For ranking purpose, we have combined the two response parameters to a single one by multiplying the power density and the thermodynamic efficiency. This response parameter is the highest (34 W/m2 %) for the Selemion (AMV and CMV) pair.