The formation of an ultrathin high performance chitosan rejection layer over a poly(ether sulfone) (PES) membrane support has been demonstrated in this study for its application in heavy metal removal by nanofiltration (NF). As a natural abundant resource, chitosan features advantages such as hydrophilicity and low cost. The aqueous chitosan solution was coated on the PES support first and partially dried in the fume hood for a certain period of time, before the introduction of hexane solution containing trimesoyl chloride (TMC) on its surface. The cross-linking reaction then took place immediately at the interface between hexane and water. The chitosan concentration and drying duration have been found to greatly affect the membrane morphology and performance. Comparisons with conventional water-soluble cross-linkers have revealed that the interfacial cross-linking by TMC is not only fast and time-saving, but also produces an ultrathin dense-selective layer of cross-linked chitosan with subnanometer pore sizes. As a result, a superior pure water permeability of 3.45 +/- 0.25 L m(-2) h(-1) bar(-1) and high salt rejections toward MgCl2, Pb(NO3)(2), and NiCl2 have been achieved by the novel NF membrane, making it highly efficient for heavy metal removal.