In the Activated Sludge Model No. 1 proposed by an IAWPRC Task Group, the possible differences of the hydrolysis rate for the slowly biodegradable organic compounds (SBCOD) under aerobic, anoxic and anaerobic conditions are accounted for by the introduction of a correction factor ηh. In the present study, this factor ηh was experimentally evaluated using starch as organic substrate to represent SBCOD. It was found that after the activated sludge had been well acclimatized with starch, the hydrolysis rate is insignificantly affected by the condition of the final electron acceptor. The value of ηh approaches unity. Results also suggested that biomass concentration has no significant influence on hydrolysis rate of starch.