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Effects of microaeration on hydrolysis of primary sludge are investigated in 500 ml batch reactors at 377 °C. Two experiments, one with a microaerobic inoculum and one with a combination of a microaerobic and an anaerobic inoculum, are carried out to also investigate the role of the inoculum. Assuming an acidogenic, methanogenic and aerobic biomass yield of 0.1, 0.05 and 0.45 mgC/mgC, respectively, a 50–60% hydrolysis increase, during the 4 day experiment, is observed with a ratio between aerobic and anaerobic metabolism in the range 0.5–0.7. The extra hydrolysed products are oxidized to carbon dioxide and incorporated into new biomass. The oxygen utilization to carbon dioxide production ratio was ∼1:1 on a mol basis. Effects of the oxygen supplied on the hydrolysis of carbohydrates, proteins and lipids are analyzed based on measurements and balances of dissolved carbon, nitrogen and COD. The total observed hydrolysis increase can be accounted for by increased hydrolysis of carbohydrates and proteins. Lipids are only hydrolysed when anaerobic inoculum is added, but no effect of oxygen availability is detected.