The distribution and transformation of molecular weight (MW) of organic matters in a pilot-scale submerged membrane bioreactor (MBR) and a conventional activated sludge (CAS) process was studied in order to better understand organic pollutant behaviours and removal mechanisms in the two processes. Gel filtration chromatography (GFC) images of the CAS process showed that the influent wastewater had relatively large MW molecules and a narrow MW distribution while the dissolved organic matters (DOM) in the anaerobic, anoxic, oxic basins and the effluent water demonstrated a broad MW distribution. MW distribution in the MBR system illustrated that the organic matters with high MW molecules were degraded into relatively low MW organic substances, whereas DOM in the oxic basin had a little part of high MW molecules produced by microorganisms during the metabolism of substrates. There is a lack of those high MW molecules in the effluent water due to the retention of the macromolecules by the membrane. Test results also showed that in both the CAS process and the MBR the majority of molecules in the influent wastewater with MW between 100 kDa and 500 kDa were mainly transformed into the molecules with MW between 10 kDa and 100 kDa. (C) 2009 Elsevier B.V. All rights reserved.