Two 50 kDa polyacrilonitrile ultrafiltration membranes, one cationic (CM) and the other anionic (AM), have been tested in order to study the removal of humic acid or natural organic matter and to examine their applicability for water purification. Experiments were carried out using synthetic water formulated from humic acids, and two natural source waters having different physical and chemical characteristics. The effect of the pressure (ranging between 100 kPa and 400 kPa), ionic strength (pI) (1.4 x 10(-3) mol L-1, 2.5 x 10(-2) mol L-1, and 4.1 x 10(-2) mol L-1; expressed as KCl) and pH (2.7,5.8,6.8,7.8 and 8.8), on synthetic water ultrafiltration, were investigated. The membranes tested produce high permeate flows, higher in the case of AM. Pressure increases do not greatly enhance permeate flows. In general, a slight decrease in flux results for an increase in pI (8% and 6% in the case of CM and AM, respectively if the pI increases from 1.4 x 10(-3) M to 4.1 x 10(-2) M). At pH 2.7, the flux reductions are in excess of 42% for both membranes, whereas at pH 8.8, the flux reduction is a minimum; 11% with CM and 4% with AM. AM undergoes a greater permeate flux reduction than CM in natural water ultrafiltration experiments, in contrast to assays with humic acid. This is probably due to the high hydrophilicity of the anionic membrane, and to the different nature (hydrophilic character in these waters) and molecular weight distributions of the organic matter. (C) 2009 Elsevier B.V. All rights reserved.