Abstract Interfacial composite membranes were prepared by dip-coating poly(vinyl alcohol) hydrogels on polysulfone ultrafiltration support membranes. Ultra-thin poly(vinyl alcohol) films were cast using multi-step coating procedure with dilute poly(vinyl alcohol) aqueous solutions and stabilized by a novel in situ cross-linking technique using five different cross-linking agents. A previously developed film casting technique allowed a constant film thickness of 200–300nm to be maintained while varying cross-linking degree and cross-linking agent. The effects of crosslinking on the molecular structure, separation performance, and interfacial properties of poly(vinyl alcohol)–polysulfone composite membranes were probed experimentally and with molecular dynamic simulations. Higher degrees of cross-linking correlated strongly with lower PVA film crystallinity and decreased hydrophilicity. Experimentally determined solvent and solute permeability data correlated almost perfectly with molecular dynamics simulated fractional free volumes of the cross-linked poly(vinyl alcohol) membranes, demonstrating the importance of polymer free volume (i.e., steric exclusion and hindered diffusion) in solvent and solute transport through nanofiltration membranes and suggesting that predictive, in silico design of cross-linked PVA coating films may be practical.