Polyelectrolyte complexes (PECs) between quaternary ammonium cellulose ether (QCMC) and sodium carboxymethyl cellulose (CMCNa) were synthesized and abbreviated to QCMC-CMCNa PECs. Ionic complexation degree (ICD(theory)) of PECs was tuned and composite PEC nanofiltration membranes (PECNMs) with different ICD(theory) were prepared in the chemical cross-linking method. Chemical structures of PECs and surface morphologies of PECNMs were characterized by Fourier transform infrared spectroscopy, field emission scanning electron microscope and atomic force microscopy, respectively. Effects of hydrophilicity and particle sizes of PECs on nanofiltration performances of PECNMs were studied. It was found that PECNM with ICD(theory) = 0.1 showed a good rejection to K(2)SO(4) of 87.7%. Effect of membrane preparation conditions and nanofiltration operating conditions on PECNM (ICD(theory) = 0.1) nanofiltration performances was studied in detail. PECNMs showed higher rejection to multi-valent anions (SO(4)(2-)) than that to monovalent anions (Cl(-)), and a highest rejection to organic dye molecules of xylenol orange. Moreover. PECNMs showed a good stability and anti-fouling capability in the long time operation. All these results indicate that PECs constitute a new family of nanofiltration membrane materials that have great application potentials. (C) 2010 Elsevier B.V. All rights reserved.