A large amount of potential renewable energy can be extracted from the mixing of freshwater and seawater. Two membrane processes can be used to extract this energy, namely pressure retarded osmosis (PRO) and reverse electrodialysis. Both processes need membranes that are similar to the ones used to perform reverse osmosis and electrodialysis. So far neither of the processes has been optimally designed. In the present study PRO has been analysed for potential power production. It appears that PRO exhibits a significant potential for power production and future developments may further improve this. A specific power in the order of 5 W/m(2) of membrane seems possible and approximately 40 per cent of the potential mixing energy of freshwater with infinite amount of seawater can be converted to mechanical energy. This article aims to describe the PRO process and to develop guidelines for the development of membranes that are suitable for PRO. Furthermore, some experimental results from laboratory measurements are included both to verify the model and to show present state of membrane development. (C) 2009 Elsevier B.V. All rights reserved.