Ion exchange membranes are core elements in reverse electrodialysis (RED), a process that can generate electricity from salinity gradients. The electrochemical and physical properties of these membranes are RED performance-determining factors. Although several studies regarding the optimization and modeling of membrane properties for RED have been performed, conclusions based on real experimental data of the relationship between physicochemical membrane bulk properties and power density (power output per unit membrane area) are still lacking. In this work, we studied bulk membrane properties of both a series of commercially available membranes and tailor made membranes and correlated these to experimental RED performance data. We successfully constructed the RED stack completely built with tailor-made membranes, made of sulfonated polyetheretherketone (SPEEK) as a cation-exchanging material and polyepichlorohydrin (PECH) as an anion-exchanging material. We obtained the highest gross power density when using only tailor-made membranes because of their significantly reduced membrane resistance. Using the experimental data, we developed a model based on multiple linear regression for gross power density which provides a better understanding on the dominant performance-determining membrane properties in RED to generate power from salinity gradients. The results set the directions towards tailoring ion exchange membranes for RED applications and show that emphasis should be especially on the development of low resistance membranes, whereas further increase of the permselectivity only has a limited effect.