Here, we report On a new concept for formation of mixed matrix reverse osmosis membranes by interfacial polymerization of nanocomposite thin films in situ on porous polysulfone supports. Nanocomposite films created for this study comprise NaA zeolite nanoparticles dispersed within 50-200 nm thick polyamide films. Hand-cast pure polyamide membranes exhibit surface morphologies characteristic of commercial polyamide RO membranes, whereas nanocomposite membranes have measurably smoother and more hydrophilic, negatively charged surfaces. At the highest nanoparticle loadings tested, hand-cast nanocomposite film morphology is visibly different and pure water permeability is nearly double that of hand-cast polyamide membranes with equivalent solute rejections. Comparison of membranes formed using pore-filled and pore-opened zeolites suggest nanoparticle pores play an active role in water permeation and solute rejection. The best performing nanocomposite membranes exhibit permeability and rejection characteristics comparable to commercial RO membranes. As a concept, thin film nanocomposite membrane technology may offer new degrees of freedom in tailoring RO membrane separation performance and material properties. (C) 2007 Elsevier B.V. All rights reserved.