Additive approach provides a simple, yet very effective way of improving the performance of polyamide (PA) thin film composite (TFC) membranes. However, the correlation between physical and chemical properties of a wide range of additives and the final membrane properties is still barely understood. In this work, a variety of additives were systematically used to prepare RO membranes to provide fundamental mechanistic understanding of membrane performance improvement by additives. The membranes were characterized in detail by FTIR, XPS, SEM, AFM, water contact angle, zeta potential, QCM etc. The separation performance of the TFC membrane was measured with 2000 ppm NaCl solution at 1.5 MPa and 25 degrees C. The results show that by adding DMSO, formamide, acetamide into the aqueous solution, or cyclohexanone into the organic phase, approximately 2 times higher water flux was achieved without significantly decreasing salt rejection. Both aqueous and organic phase additives bring out better MPD diffusion and higher crosslinking density. However, their effect on the kinetic of interfacial polymerization are different. The aqueous phase additives promotes the formation of a thinner and smoother PA selective layer with less film mass, while the organic phase additives sustain the solubility of PA oligomers so that the film mass increases and greater film mass correlates with improved membrane flux. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.