Insitu Raman spectroscopy has been used to show that peroxide ions are present on Ba/MgO catalysts at temperatures up to 800 degrees C. The surface O-2(2-) ions are believed to be responsible for the activation of CH4 during the oxidative coupling reaction which occurs at these elevated temperatures. At 100 degrees C the peroxide ion spectrum is characterized by a major line at 842 cm(-1), with minor peaks at 829 and 821 cm(-1). As the temperature was increased, all of the peaks shifted to lower wavenumbers and broadened, until at 800 degrees C only a broad asymmetric band remained. Introduction of O-18(2) also caused the peaks to shift to lower wavenumbers, consistent with the assignment of the spectra to peroxide ions. In the presence of CO2 and H2O the O-2(2-) ions reacted to form carbonate ions at T greater than or equal to 500 degrees C, which is in agreement with the poisoning effect of CO2 in the catalytic experiments. At Ba loadings less than or equal to mol % a two-dimensional layer of BaO2 is present on the surface of MgO, but at larger loadings crystallites of BaO2 contribute significantly to the Raman spectra.