Abstract The idea to generate power through osmosis between river and ocean water has been known since the 1970s. The potential power that can be produced worldwide through osmotic power is estimated to be 1600 TWh/a. But due to inefficient membranes, little effort has been put into research for this type of renewable ocean energy. In 2001, Statkraft, one of the major energy providers in Norway, invited GKSS-Forschungszentrum to develop a suitable osmosis membrane for pressure-retarded osmosis (PRO). Two different types of membranes were optimised: thinfilm composites (TFC) and asymmetric cellulose acetate. To make PRO profitable, the power density of the membrane was determined to be between 4–6 W/m2. Starting with power production from 0.1 W/m2 for the TFC membrane, a power density of 3.5 W/m2 with a potential of 5 W/m2 was measured. The starting value for the CA-type was approximately 0.5 W/m2, and the best measured performance was 1.3 W/m2. However, if it is possible to improve PRO membranes further, PRO will move to the idea of a profitable application, generating green, emission-free energy.