In this study the effect of preozonization of the secondary effluent on the fouling of ultrafiltration membranes was investigated. The results showed that preozonization could effectively improve the biodegradability of secondary effluent, the apparent molecular weight distribution (AMWD) of organic substances in water was different in different ozone contact time, the AMWD of organic substances had great influence on membrane resistances, blocking and cake layer resistances had controlling force over fouling resistance. The following was the relationship between AMWD of organic substances and membrane resistances: (i) when d/phi < 2/15 (d: molecular weight of the organic substances in water, phi: membrane pore size), organic substances could easily permeate membrane pore, and when 2/15 < d/phi 1/3, organic substances tended to be adsorbed inside of membrane pore; (ii) when 1/3 < d/phi < 1, organic substances mainly caused interior membrane pore blocking, and when 1 < d/phi < 5/3, organic substances mainly caused the block in membrane surface; (iii) when d/phi < 1, especially when d/phi < 5/3, organic substances mainly formed membrane surface cake. (c) 2006 Elsevier B.V. All rights reserved.