A basic approach to the problem of activated sludge synthesis and substrate oxidation has been taken by using the thermodynamic concept that free energy change is a measure of the maximum work of which a process is capable. As a consequence, synthesis is proportional to substrate free energy of oxidation. The relationship was verified experimentally and by means of computations using data from the literature. Calculations also showed synthesis as being proportional to substrate COD. Finally, derivation indicated that the fraction of substrate oxidized is a function of the ratio of the COD to the carbon content of the substrate.