The hydrolysis of settle able substrate in domestic sewage was evaluated using its O-2 utilization rate profile generated in an aerated batch reactor. The hydrolysis rate coefficient was 1.2 d(-1), significantly lower than 3.8 d(-1) and 1.9 d(-1) characterizing soluble and suspended slowly biodegradable fractions. The settled portion of the sewage incorporated an active biomass fraction of 1560 mg COD l(-1) that needed to be accounted for in the hydrolysis kinetics. The texture of the biomass/settled substrate mixture and the gradual hydrolysis of the particulate substrate within the floc structure were examined by microscopic analysis.