The main problem preventing the widespread use of membrane bioreactors (MBR) for wastewater treatment is membrane fouling. In this study, we proposed ozonation as a new method for controlling membrane fouling in MBR. Through a batch test, we found that membrane filterability of the mixed liquor was improved by ozonation with a dosage less than 0.7 mg/g-SS. In a further experiment, an ozonation-coupled MBR and a conventional MBR were run for over 30 days twice, which confirmed the long-term effectiveness of ozone on fouling control. Chemical oxygen demand and ammonia removals in the ozonation-coupled MBR were not affected by ozonation. From the batch test, effects of varying ozone dosages on the mixed liquor properties were measured. When the dosage was less than 0.7 mg/g-SS, extra-cellular polymeric substances were reduced, leading to a slight increase of supernatant organic concentration and more hydrophobic flocs' surface. Moreover, the zeta-potential of colloids was simultaneously decreased. Enhanced hydrophobicity of the flocs' surface and decreased zeta-potential of colloids provided a beneficial condition for re-flocculation among bioflocs. After the ozonated mixed liquor was aerated, the flocs re-flocculated. Floc size increased while supernatant organic concentration decreased. These were responsible for improved membrane filterability of the mixed liquor by ozonation. (C) 2008 Elsevier B.V. All rights reserved.