Polyethersulfone ultrafiltration membranes (MWCO: 9000, 35,000 and 85,000) were surface modified by preadsorption of poly(sodium 4-styrenesulfonate) (PSS) upon the permeation of aqueous solution of the polymer for about 100 min. Membranes with lower MWCO values were modified primarily on the top surface, whereas high MWCO membranes were modified both on the surface and pore walls as well. The IR spectra of surface modified membranes show absorption bands at 1040 cm(-1) due to symmetric vibration Of SO3- group, and at 2925 and 2852 cm(-1) corresponding to asymmetric and symmetric stretchings, respectively, of aliphatic (backbone CH2 and CH) unit present in PSS. Surface modified membranes have shown separations of 28-56% for Na2SO4, 11-19% for NaCl and < 5 % for CaCl2 with water permeation rates of 35-102 lm h at 2 kg/cm(2) when tested for 500 ppm feed solution. The rejection of Na2SO4 decreases in an exponential manner when the concentration of Na2SO4 in the feed solution was increased from 500 to 2500 ppm, which is a characteristic of negatively charged membranes. The antifouling nature of unmodified and surface modified membranes were compared by ultrafiltration of aqueous solutions of different molecular weight polyethylene glycols and dextrans. Surface modified membranes have shown better antifouling properties compared to unmodified membranes, as indicated by flux decline with operating time.