Ba/MgO is an active catalyst for the oxidative coupling of methane to form ethane and ethylene. It has been proposed that activation of methane occurs via reaction with peroxide species present at the surface of the catalyst. In the present work, Raman spectroscopy has been used to investigate the formation, decomposition, and reduction of BaO2 on 4 mol% Ba/MgO. The presence of BaO2 is evidenced by the presence of a band at 842 cm(-1). The peroxide forms above 300 degrees C but is stable to decomposition at temperatures up to 500 degrees C. Reduction of BaO2 to BaO proceeds via Ba(OH)(2). BaCO3 forms when either BaO or BaO2 is exposed to CO2. Once formed, BaCO3 is stable to decomposition in He or O-2 at temperatures up to 500 degrees C. Only BaCO3 is observed when a mixture of CH4 and O-2 is passed over the catalyst at 500 degrees C.