Influence of organic matter (OM) on uranium removal mechanisms by ultrafiltration (UF) over a pH range of 3-11 was investigated. Humic, alginic and tannic acid were used as OM. it was found that uranium adsorbed strongly to the membrane while retention by size exclusion did not occur. Adsorption was dependent on pH and type of OM used. Speciation predictions performed using Visual Minteq explain some of these results. In the absence of OM, uranium primarily adsorbed to the membrane at pH 5 and 7 where UO(2)OH(+) and UO(2)CO(3) were the dominant species. In the presence of humic acid (HA), uranium adsorption increased in the acidic range, especially at pH 3 (from 11% to 74%) due to the complexation. The structure of alginic acid (AA) did not favour complexation with uranium and therefore did not have a significant influence on its behaviour in UF. The exception was at pH 3 where adsorption increased from 11% to 52%. At this pH no charge repulsion between the uranium species and the AA occurs and complexation is favoured. The highest effect on uranium adsorption was obtained in the presence of tannic acid (TA) at pH 10 and I I where adsorption increased from 20% up to 100%. Uranium is most likely forming complexes with the gallic acid fraction of the dissociated TA. (C) 2009 Elsevier B.V. All rights reserved.