Lanthanum oxide catalysts have been studied by X-ray diffraction under conditions used for the oxidative coupling of methane. All catalysts were prepared by coprecipitation of lanthanum nitrate with ammonium carbonate and calcined at temperatures between 650-degrees-C and 1200-degrees-C before testing. Lanthanum oxide forms both type I and type II lanthanum oxycarbonate under methane coupling conditions. However, the type II phase is the more stable at high temperatures and is the only one that exists at reaction temperature (800-degrees-C) after the first two hours on stream. High-temperature calcination changes the morphology of the catalyst and reduces its total surface area which is concurrent with a decrease in the amount of type II oxycarbonate formed under reaction conditions. The formation of oxycarbonate may occur preferentially on the (001) faces of platelets formed by calcination at low temperature. However, when the catalysts are tested under conditions which preclude the total consumption of reactants, at space velocities of 1 290 000 h-1, little difference is observed in the activity of these catalysts or in their selectivity to C2 products. There does not seem to be a simple relationship between the amount of bulk oxycarbonate formed on the catalyst and its selectivity to C2 products during the oxidative coupling of methane.