Poly (N-vinyl imidazole) (PVI) gel filled nanofiltration (NE) membranes were synthesized by UV-initiated graft polymerization of N-vinyl imidazole (VI) monomer, with and without crosslinker N,N'-methylenbisacrylamide (MBAA) on/in a polysulfone (PSI) ultrafiltration (UF) substrate. A comprehensive characterization of the resulting composite membranes was conducted in terms of surface chemical composition (ATR-FTIR, XPS), surface morphology (SEM), surface charge (zeta potential), surface wettability (water contact angle) and barrier properties (water permeance, neutral solute/ions rejection and membrane stability). From rejection data of neutral solutes, including Vitamin B-12 and PEGs, the average pore sizes and pore size distributions of membranes were further determined. These results confirmed the prepared membranes as stable, defect-free, smooth, hydrophilic NE membranes with effective pore size between similar to 1.5 and 2 nm and water permeance between similar to 0.5 and 10 L m(-2) h(-1) bar(-1), Moreover, owing to the weak polybase characteristic of the incorporated PVI gel, the membranes would carry slight positive charge in neutral or slightly acidic solution. The small pore size membranes displayed high salt rejections in the order of MgCl2 > NaCl approximate to MgSO4 > NaSO4 at near-neutral condition (pH 5.9 +/- 0.3). This testified their NE separation capability based on size exclusion, facilitated by Donnan repulsion. At last, by comparison with another reported type of gel-filled NE membranes with microporous substrate, and by applying a gel model, we delivered a qualitative description of the underlying fundamental parameters controlling the performance of such gel-filled membranes. Overall, these novel weak polybase gel-filled NF membranes with desirable nanometerssize pores and potentially tunable charge characteristics could offer great versatility for molecular-level separations. (C) 2015 Elsevier B.V. All rights reserved.