The novel positively charged poly(ether ether ketone)s (PEEKs) with pendant quaternary ammonium groups were synthesized by copolymerization of 3, 3'-dimethylaminemethylene-4,4'-biphenol (DABP), 3,3',4,4'-tetramethylbiphenol, and 4,4'-bisfluorobenzophenone followed by reaction with iodomethane. The resulting copolymers were used to prepare thin film composite (TFC) nanofiltration (NF) membranes via the dip-coating method. The effects of different parameters such as copolymer concentration and curing time on the membrane performance (flux and rejection of inorganic salts) were investigated. The optimum parameters were that 1.5 wt % quaternary ammonium PEEK containing 1.8 quaternary ammonium groups per unit with 0.5 wt % DMSO coated on the polysulfone (PSf) support membrane and cured at 100 degrees C. The results of the performance testing showed that the trend for rejection was R-MgCl2 > R-CaCl2 > R-NaCl > R-Na2SO4 (R = rejection), which was a typical positively charged membrane. The best performance of these composite nanofiltration membranes was 91.3% rejection for 500 ppm MgCl2 and 62.5 L/m(2) h water permeability at 0.4 MPa. The MWCO of the membrane was 800 Da. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 127: 1601-1608, 2013