The effect of oxygen concentration on simultaneous sewage sludge digestion and metal leaching (SSDML) process was studied in 30 l reactors. Sewage sludge from Black Lake (Quebec) wastewater treatment plant was used. Concentrations of 1, 2, 3, 5 and 7 mg/l of oxygen were maintained successively in each of the reactor experiments. The increase in the DO concentration from 2 to 7 mg/l in the sludge resulted in an increase of the oxidation-reduction potential (ORP) and an increase in the acidification rate and degradation of the total volatile solids (VS) in the sludge. The pH reduction was directly related to the dissolved oxygen (DO) concentration in the sludge. This marked decrease of pH resulted in significant solubilization of metals (Cd: 17.32-47.03%; Cr: 9.12-18.00%; Cu: 41.53-80.72%; Ni: 26.87-56.75%; Pb: 9.12-17.60%; Zn: 48.01-92.98%). There was also a reduction of VS up to ~ 50%. This method allowed the sludge to retain its fertilizing properties intact. Nonetheless, after 14 days of treatment, the pH value, the percentage removal of the VS and the percentage solubilization of metal obtained were practically the same for tests involving 2, 3, 5 and 7 mg/l DO. After the first 4 days, the air required in each reactor in order to maintain the same initial DO concentration was less. In the cases of tests using 3 and 1 mg/l DO, the system was operated with aeration rates of 0.2 vvm (volume of air/vol of sludge in reactor/min) to 0.03 vvm and 0.18?0.01 vvm, respectively.