Composite nanofiltration membranes were prepared by coating poly(vinyl alcohol) hydrogels on polysulfone ultrafiltration support membranes. Ultra-thin and defect-free poly(vinyl alcohol) hydrogels were cast using multi-step coating procedure with dilute PVA aqueous solutions and novel in situ cross-linking. The combined Spiegler-Kedem-film theory model was used to extract water permeability, solute permeability, reflection coefficients, and mass transfer coefficients for the composite membranes. Transport of water and salt ions through PVA coating films was dramatically influenced by feed solution pH and counter-ion valence as well as PVA molecular weight, concentration, and extent of cross-linking. Characterization of PVA coating film thickness, extent of cross-linking, surface thermodynamic properties, and crystallinity were used to explain differences in observed transport properties of the composite membranes. (C) 2010 Elsevier B.V. All rights reserved.