Two pilot-scale membrane plants were set up to produce drinking water, and membrane backwash water was discharged during the production process. This work studied the characteristics of dissolved organic matter (DOM) in membrane backwash water from submerged microfiltration (MBWS) and pressurized ultrafiltration (MBWP) both of which are coupled with the pre-coagulation process. The results showed that the two waters had similar molecular weight (MW) distributions. Dissolved organic carbon (DOC) and trihalomethane formation potential (THMFP) in MBWS and MBWP were both mainly distributed in MW > 30 kDa and MW < 1 kDa. and UV254 was mainly in MW < 1 kDa. For Luan River water (LRW, the raw water for the two pilot-scale membrane plants in this study), organic matter enriched in membrane backwash water was mainly in sizes of MW> 30 kDa. In addition, organic matter with MW > 10 kDa was higher in MBWP than in MBWS. The quality of membrane backwash water was influenced by the changes in LRW quality during different periods. The quality of membrane backwash water was worse in alga-laden period than in normal period and organic matter concentrations in MW < 1 kDa increased significantly in this period. The small size DOM in membrane backwash water was more reactive to form trihalomethanes (THMs) in the disinfection process. The variability of specific UV absorbance and THMFP/DOC was consistent in membrane backwash water. (c) 2009 Elsevier B.V. All rights reserved.