The total solubility of oxygen was measured at 650°C for electrolyte. The chemical method used for analysis enabled the determination of the sum of the concentrations of molecular oxygen, superoxide ion, and peroxide ion. However, by varying the and partial pressures, the major oxygen component was found to be superoxide, with the concentration of peroxide being considerably smaller (perhaps negligible) and that of molecular oxygen negligible. These data are pertinent to the study of oxygen reduction at fuel cell cathodes.