Dewatered sewage sludge (about 84% moisture content, organic content: 33.4% protein, 6.6% lipid and 3.3% carbohydrate on an organic basis) was liquidized by a thermal process. The liquidized sludge, which has a high organic concentration [11.4% volatile solids (VS) concentration], was separated by centrifugation into a liquid and solid phase, and then the organic compositions of the liquidized sludge and its centrifuged phases (liquid phase and solid phase) were analyzed. The amount of proteins in the liquidized sludge (27.6%) decreased through the liquefaction, but the ammonium concentration increased. The amount of lipids (13.8%) in the liquidized sludge increased. These results show that proteins were hydrolyzed and degraded to volatile acids and ammonia via amino acids. The liquid phase (6.9% VS concentration) contained suitable substrates (15.5% lipid and 9.2% carbohydrate) for anaerobic digestion. These results show that the anaerobic digestion of the liquid phase could contribute to a higher digestion ratio and greater methane yield. The solid phase, which contained low moisture (77.0%), could be available for composting. Copyright (C) 1996 Elsevier Science Ltd.