THE method proposed by Alcock1 as a means of correcting an approximate free-energy change calculated from the simple two-term equation leads to a system of thermochemical data storage which possesses many advantages over the free-energy function and polynomial methods. This “free-energy deviation” does not, however, allow for transformations or changes in state of reactants and products, whereas in many processes the standard free-energy change would be required for examples such as to provide the basis for calculating equilibrium activities in solution and equilibrium partial pressures in a vapour phase. It is immediately apparent that the heat and entropy changes associated with such transformations must be taken into account and for reaction (1), where it is assumed that the solid state represents the normal equilibrium form of the metal and its oxide at 298° K, an approximate free-energy change is given by For many purposes this approximate free-energy change is sufficient, but in the terms in which it is written it cannot be combined with one or more free-energy deviation functions to give a true value for the change in free-energy.