The process of thermophilic aerobic digestion has been proposed to treat wastewater sludge, either alone or mixed with garbage from household garbage disposals. The major reactions involved in the process are presented, discussed, and simplified into an overall net reaction for the determination of the basic stoichiometric, thermochemical, and kinetic relationships. Volatile solids destruction, oxygen utilization, and carbon dioxide production are related stoichiometrically for both ammonia and nitrate as products of organic nitrogen oxidation. A simple procedure is presented for determining the thermochemistry of the net reaction by relating heat production to the oxygen equivalent or COD of the organic solids. The simulation studies and mathematical model discussed employ conservative values of biological parameters. The mathematical model provides assistance in planning and conducting pilot plant studies.