Equations are constructed representing the growth of Saccharomyces cerevisiae in batch culture anaerobically on glucose, and aerobically on glucose, ethanol, and acetic acid. The mass of cells formed during a growth process is defined in terms of an ion-containing carbon mol (ICC-mol) of cells, the thermodynamic properties of which have been determined. Because the process of growth is completely irreversible, the free energy change accompanying a growth process cannot be measured or calculated directly. It was, therefore, necessary to obtain data from direct calorimetry for the heat of combustion of 1 ICC-mol of whole dried cells (not ash free) and from this to calculate its heat of formation. It was also necessary to determine, by direct calorimetry, the entropy of this mass of cells, and from this to calculate its entropy of formation. With these two values, the foe energy of formation of 1 ICC-mol of cells was calculated with the Gibbs free-energy equation. These thermodynamic properties were then used together with those of the reactants and products of a given growth process to calculate, by indirect calorimetry, the thermodynamic changes accompanying the growth of S. cerevisiae on the substrates used. Considerations are made as to what growth is most closely related. It is concluded that the quantity of electrons conserved within the biomass produced during growth is the most accurate indication of the efficiency of energy conservation, all other energy changes being calculable from such data.