The effect of microwave irradiation on preparation of composite nanofiltration membranes was investigated. Interfacial polymerization technique was employed by applying trimesoyl chloride (TMC) and piperazine (PIP) as the reagents for preparation of polyamide (PA) skin layer on a polyethersulfone (PES) support. The different times (0, 10, 30 and 60 s) and powers (540, 720 and 900W) of microwave irradiation were applied. The microwave effect was investigated in two states: (i) the organic solution, which contains TMC regent, was poured off from the surface and (ii) the organic solution was remained on the membrane surface. For membrane characterization, water permeation, salt rejection, chemical structure and membrane morphology were elucidated. The results showed that under an operational pressure of 1.0 MPa, and 540 W for 0, 10, 30 and 60 s irradiation for category i membranes, fluxes of 13, 8, 8, and 10 kg/m(2) h and 46, 63, 57, and 51% retention for NaCl and 82, 88, 87, and 88% rejection of Na2SO4 were obtained. The rejection results for category ii were approximately similar to the first category. Increasing the irradiation power resulted in a slight decline in the rejection. The chemical composition and the surface image of membranes were obtained by using ATRAR, EDX, SEM and AFM. (C) 2009 Elsevier B.V. All rights reserved.