For thin film composite (TFC) reverse osmosis (RO) membranes, a trade-off is evident between water permeance and salt rejection; and yet to improve their RO performance it is desirable to increase flux without sacrificing selectivity. This paper describes the fabrication of polyamide TCF membranes on highly solvent stable crosslinked polyimide UF supports and the use of an activating solvent as a strategy to increase water flux; and in certain cases, water permeance and salt rejection are increased simultaneously. Performance was evaluated in terms of NaCl rejection and water permeance before and after post-treatment with dimethylformamide (DMF) as the activating solvent. Additionally, the morphology of the TFC membranes was characterized by SEM, their hydrophilicity was determined by contact angle measurements, and zeta (xi)-potential was obtained by a streaming potential based method. NaCl rejection was found to be improved for TFC membranes with initial NaCl rejections below 90%, reaching a maximum value of 94%. Water permeance was also increased and to greater extent for membranes with low initial permeance values. The biggest improvement was obtained for a TFC membrane with initial permeance value of 0.2 L.m(-2).h(-1).bar(-1), rising to 1.6 L.m(-2).h(-1).bar(-1) with a rejection of 93.8%. (C) 2014 Elsevier B.V. All rights reserved.