Chemical modification was employed for tuning the molecular weight cut off (MWCO) values of poly(acrylonitrile-co-methacrylic acid) hollow fibre ultrafiltration membranes by treating them with aqueous NaOH solutions of different concentration (0.25, 0.75, 1.25 and 1.75 M) separately for 1, 3, 6 or 9 h and then with 0.1 M HCl solution for 2 h at room temperature. This has resulted in the decrease of MWCO values of the fibres from the initial 92 kDa to the range of 9-75 kDa. The flux was decreased from 215 to 28-150 L/m(2) h at 1.5 kg/cm(2). NaOH-HCl treated fibres exhibited somewhat higher MWCO value and permeate flux than those of the fibres which were treated with NaOH only. Modified fibres exhibited lower fouling tendency for the ultrafiltration of aqueous solutions of polyethyleneoxide and bovine serum albumin, as indicated by their flux recovery ratio values of 91%-93%, when compared to the 53%-58% for virgin fibres. IR spectra of NaOH treated fibres showed a new peak at about 1580 cm(-1) due to the conversion of nitrile group to cyclic -C-N- naphthyridine structure. HCl treatment was resulted in the disappearance of the peak at 1580 cm(-1) and appearance of a new peak around 1695 cm(-1) due to the amide carbonyl group that was generated by the hydrolysis of naphthyridine structure. The fibres were characterized by scanning electron microscopy, contact angles and zeta potential measurements. (C) 2012 Elsevier B.V. All rights reserved.