Three different energy concepts of cell yield are investigated using synthesis data measured in mixed aerobic cultures for 15 different substrates. These include, the energy-synthesis concepts of McKinney, Servizi and Bogan, and McCarty. Growth, substrate removal, and oxygen uptake measurements at 25○ C. are correlated with various thermodynamic functions including enthalpy and free energy. Efficiency measurements based upon free energy computations agree with McCarty's results. However, the study results conflict with the magnitude of cell yield per unit change of free energy of oxidation as estimated by Servizi and Bogan. Conclusions regarding the validity of McKinney's heat loss-cell yield relationship were not possible although the proportionality between heat of reaction and oxygen utilization was verified. This latter relationship is based upon hypothetical oxidative assimilation equations computed from the experimental data and assumed protoplasm formulations. Substrates used in the study included compounds representing the basis food substances of carbohydrates, fatty and amino acids.