This paper studied in detail the sludge disintegration and supernatant changes during ozonation and investigated the possible mechanisms. Ozone effectively lysed the sludge. The most important mechanism is proposed to be damages of microorganism cells. The ozone dose of 50 mgO(3)/gDS was found optimal for sludge lysis; 25 MgO(3)/gDS achieved only 10.4% Sludge lysis after 90 min and 80 mgO(3)/gSS did not further improve sludge decomposition. When the ozone dose was 50 mgO(3)/gDS, the sludge disintegration degree was 46.7% after 105 min. The sludge solid concentration and volatile solid concentration decreased by 49.1% and 45.7%, respectively. The supernatant soluble chemical oxygen demand, total nitrogen, total phosphorus, protein, polysaccharide, and deoxyribonucleic acid increased by 699%, 169%, 2379%, 602%, 528%, and 556%, respectively. Various components showed quite different patterns. Ozone treatment significantly reduced the sludge bioactivity, decreased the partition coefficients of heavy metals between sludge and supernatant, but did not alter the sludge size distribution. (C) 2008 Elsevier Ltd. All rights reserved.