A comparison has been made of the behaviour in the oxidative coupling of methane of the oxides of Sm, Dy, Gd, La and Tb with that of a Li/MgO material. All but the Tb4O7 (which gave total oxidation) were found to give higher yields than the Li/MgO material at temperatures up to approaching 750°C but the Li/MgO system gave better results at higher temperatures. The cubic structure of Sm2O3 was found to be responsible for its good performance while the monoclinic structure was relatively inactive and unselective. The addition of Na or Ca to cubic Sm2O3 gives a higher optimum C2 yield than that of unpromoted Sm2O3. Sm2O3 and Ca/Sm2O3 catalysts are more stable than Li/MgO, Li/Sm2O3 or Na/Sm2O3. The addition of Li or Na to Sm2O3 causes the structure to change from cubic to monoclinic; the deactivation of the Na/Sm2O3 catalysts is caused by a loss of Na coupled with the formation of the monoclinic form of Sm2O3.