A method was developed for computing the partial pressures of the various equilibrium vapor species existing above nonstoichiometric UO2 + x. The gas phase oxygen-to-uranium atom ratio was correlated with the oxygen-to-uranium ratio in the solid, which was known. The atom ratio, total pressures, and equilibrium constants of formation of each compound, sufficiently specify the system to allow the computation of the gas phase concentrations using multicomponent chemical equilibria computer techniques. The results agree with independent experimental data.