The hydrolysis of settleable substrate in domestic sewage was evaluated using its O₂ 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.