Polyamide composite nanofiltration membranes are fabricated under constricted polymerization time. The membranes were characterized for pore size, effective thickness/porosity as well as effective membrane charge density using the Donnan Steric Pore Flow Model. The effect of polymerization (reaction) time on the membrane characteristics and membrane performance (flux and rejection) was studied. The effect of polymerization time on pore size is not significant for reaction times less than 120 s but the effective thickness/porosity seems to be increasing proportionally with polymerization time. Effective membrane charge density for all the membranes depended on the degree of condensation and hydrolysis process. Polymerization time is an important process condition that could change the effective thickness/porosity, pore size, and effective volume charge density and subsequently affect the membrane performance in terms of flux and rejection. (C) 2004 Wiley Periodicals, Inc.