Abstract Ethylenediamine cored poly(amidoamine) (PAMAM) has recently emerged as an alternative monomer in synthesizing polyamide thin film composite (TFC) membranes through interfacial polymerization due to its dendritic and hydrophilic nature. Despite the various PAMAM contents used, the membrane separation performances were marginally improved which can be ascribed to the larger PAMAM free volume. In view of this, piperazine, which has a smaller molecular size, was introduced as co-polymerized monomer during interfacial polymerization. Piperazine plays its role as void-filler and hence will produce a more compact structured membrane. The membranes׳ physicochemical properties were characterized using Scanning Electron Microscopy (SEM), Attenuated Total Reflectance-Fourier Transform Infra-red (ATR-FTIR), X-ray Photoelectron Spectroscopy (XPS) and zeta potential, while their performance was evaluated in terms of pure water permeability and inorganic salt rejection. The mixed monomer TFC membranes had synergistically improved permeability and rejection up to 0.3% (w/v) PAMAM content. Top surface of the TFC membrane was mainly dominated by PAMAM–TMC when higher PAMAM concentration was used during the reaction. The reaction is diffusion controlled due to the dense selective layer as well as low molecular diffusivity.