In situ FTIR spectroscopy was used to characterize the oxygen adspecies and its reactivity with CH4 over LaOF and 15 mol% BaF2/LaOF catalysts at OCM temperature (750-800 degrees C). It was found that gas-phase oxygen was activated on the surface of LaOF and 15 mol% BaF2/LaOF, which had been pretreated under vacuum at 750 or 800 degrees C, forming O-2(-) species at high temperature (750-800 degrees C). At 750 degrees C, the adsorbed O-2(-) species can react with pure CH4 accompanied by formation of gas-phase C2H4 and CO2, and there is a good correlation between the rate of disappearance of surface O-2(-) and the rate of formation of gas-phase C2H4. The O-2(-) species was also observed over the catalysts under working condition, and it reacted with CH4 in a manner that was consistent with its role in a catalytic cycle. These results suggest that O-2(-) may be the active oxygen species for OCM reaction over these catalysts.