Nanofiltration has been studied and operated as an advanced water treatment process with respect to the removal of natural organic matter (NOM) and micropollutants throughout the world. However, fouling is an inherent phenomenon that reduces membrane performance, such as the flux and removal efficiency. In this study, filtration with a semi-pilot type of nanofiltration was conducted with coagulation/sedimentation treated water for the removal of NOM at a drinking water treatment plant in Korea. However, severe flux decline and reduction of removal efficiency were observed after 3 months. Tested membranes were autopsied using cleaning agents, including pure water and caustic solutions. The foulants eluted with the cleaning agents exhibited relatively high molecular weight (MW) distributions, as determined by a high performance size exclusion chromatography method, and also contained polysaccharide, and/or protein-like substances identified from IR spectra analysis. To identify the compounds with high MW values, a dialysis membrane bag, with MWCO range from 12,000 to 14,000 Da, was used to isolate the colloidal NOM from raw surface water from the Dongbok Lake in Korea. The isolated compounds were compared to various organic compounds, including algogenic organic matter (AOM), bovine serum albumin, humic acids from the Suwanee River and soluble microbial products (SMP), in terms of their size, structure, IR analysis and pyrolysis gas chromatography-mass spectrometry analysis. The colloidal NOM was highly hydrophilic, as identified from XAD resin fractionation method. The organic foulants from the tested NF membranes, with high MW and hydrophilicity, and the saccharides and/or proteins-like substances (i.e., colloidal NOM) had similar characteristics to microbial by-products, such as SMP and AOM. (c) 2005 Elsevier B.V. All rights reserved.