Fouling is one of the major challenges to be solved in order to commercialise osmotic power generation by pressure retarded osmosis (PRO). This paper deals with natural organic matter (NOM) fouling. The impact of i.a. NOM concentration and ionic strength in the freshwater feed have been investigated for five different types of PRO membranes by performing fouling experiments in PRO mode at isobaric conditions. An important finding was that fouling propensity in terms of relative flux decline as a function of accumulated NOM load was independent of the NOM concentration at otherwise identical conditions. Further, it was confirmed that increased ionic strength in the support structure, due to internal concentration polarisation and reverse salt diffusion, aggravates membrane fouling in PRO. The observed variation in fouling propensity between different types of PRO membranes was significant. The CA membrane was less susceptible to NOM fouling than the four TFC types of PRO membranes. It is anticipated that the variation in fouling propensity that were observed for different membranes was related to both material and structural properties of the membranes. The results show the importance of applying fouling propensity as an additional main criterion to membrane performance during future research and development of PRO membranes. (C) 2013 Elsevier B.V. All rights reserved.