The oxygen nonstoichiometry of La0.2Sr0.2CaO3-delta has been determined as a function of oxygen partial pressure and temperature using a high-temperature coulometric titration cell. For each measured value of the oxygen chemical potential, the oxygen nonstoichiometry is found to be nearly independent of temperature. The equilibrium partial energy and entropy associated with oxygen incorporation have been determined as a function of oxygen nonstoichiometry and temperature. The results are interpreted in terms of a model in which it is assumed that conduction Electrons, created during vacancy formation, gradually fill electron states in a wide electron band. A new relation between vacancy concentration, temperature. and oxygen partial pressure has been formulated which does not have the familiar appearance of a mass action type of equation.