Natural organic matter (NOM) has been identified as a major factor affecting membrane processes performances, but its impact is difficult to quantify from global parameters such as organic carbon content. The extent of fouling due to the different fractions of NOM from surface water has been examined in dead-end ultrafiltration using criteria such as flux decline and irreversibility in regard with organic matter rejection. The most important flux decline was observed during the filtration of the hydrophilic acids fraction whereas fulvic acids led to the most irreversible fouling. Furthermore. the hydrophilic fraction lost its fouling character when mixed with other fractions underlining that interactions between numerous components are possibly more important than the composition itself. (C) 2009 Elsevier B.V. All rights reserved.