Osmotically driven membrane processes, such as forward osmosis (FO) and pressure-retarded osmosis (PRO), rely on the utilization of large osmotic pressure differentials across semi-permeable membranes to generate water flux. Previous investigations on these two processes have demonstrated how asymmetric membrane structural characteristics, primarily of the support layers, impact water flux performance. In this investigation we demonstrate that support layer hydrophilicity or wetting plays a crucial role in water flux across asymmetric semi-permeable membranes. The results show that the polyester(PET) non-woven and polysulfone supports typically present in thin-film composite (TFC) reverse osmosis (RO) membranes do not wet fully when exposed to water, thereby resulting in a marked decrease in water flux. A cellulosic RO membrane exhibited modestly higher water fluxes due to its more hydrophilic support layer. Removal of the PET layers from the cellulosic and TFC RO membranes resulted in an increased water flux for the cellulosic membrane and very little change in flux for the TFC membrane. Pretreatment with hydraulic pressure (RO mode), feed solution degassing, and use of surfactants were used to further elucidate the wetting mechanisms of the different support layers within each membrane. The importance of considering membrane support layer chemistry in further development of membranes tailored specifically for osmotically driven membrane processes is discussed. (C) 2008 Elsevier B.V. All rights reserved.