Thin film composite (TFC) nanofiltration membranes were fabricated by interfacial polymerization using polymeric amine polyethylenimine (PEI) and monomeric amine piperazine (PIP) as the amine reactant. Membranes with a single-ply polyamide layer were produced by reacting trimesoyl chloride (TMC) with mixed amines of PEI and PIP, and incorporation of a small amount of PIP in PEI was found to increase the permeation flux effectively while still maintaining a good solute rejection. For instance, adding 10 wt% PIP in the amine reactant solution resulted in a 6-fold increase in permeation flux, while a 91.6% MgCl2 rejection was maintained. In addition, 2-ply polyamide membranes were also prepared by two cycles of PEI-TMC and PIP-TMC interfacial reactions separately, and they showed a higher rejection than the single-ply polyamide membrane. At a low PIP/PEI concentration ratio, the single-ply polyamide membranes formed with mixed amines of PIP and PEI tended to be more permeable than the 2-ply polyamide membranes. However, it was demonstrated that by properly controlling the PIP/PEI concentration ratio, the 2-ply polyamide membranes with both a higher permeation flux and salt rejection than conventional single-ply polyamide membranes could be produced. The resulting membranes were characterized for chemical composition, surface hydrophilicity, surface charge and morphology of the polyamide skin layer. (C) 2014 Elsevier B.V. All rights reserved.