Organic foulants obtained from ultrafiltration (UF) and reverse osmosis (RO) membranes of a large scale municipal water reclamation plant were rigorously characterized using conventional and advanced characterization analyses (e.g. pyrolysis and mass spectrometry) in order to identify major constituents of the organic foulants and investigate fouling characteristics in a large scale application of the UF and RO membranes. Although water qualities and characteristics of effluent organic matter in the feed water were slightly changed during the UF membrane, fouling characteristics of the UF and RO membranes used in a large scale municipal water reclamation plant were significantly different according to the type of membranes. Hydrophobic fractions comprising of carboxylic acids and aldehydes strongly contributed to the fouling formation of the UF membrane compared to that of the RO membranes whereas the RO membrane foulants mainly consisted of hydrophilic fractions comprising of amides and alcohols due to the repulsive electrostatic interaction between negatively charged RO membrane surfaces and hydrophobic fractions with a negative charge, indicating that the membrane characteristics could play an important role in the fouling formation of the tested UF and RO membranes. (C) 2012 Elsevier B.V. All rights reserved.