Aerobic digestion at thermophilic temperatures has been examined comparing air and oxygen as aeration gases. Theoretical considerations indicated that autothermal operation can only be economically achieved when high purity oxygen is used. Heat balances using oxygen and air demonstrate that when air is used, evaporative and gas sensible heat losses become substantial and thermophilic operating temperatures can not be autothermally achieved. Pilot plant testing has confirmed the operational feasibility of autothermal thermophilic aerobic digestion using high purity oxygen.