It has been found that it is possible to improve the activity and stability for the oxidative coupling of methane of a Li/MgO catalyst by the addition of small amounts of the oxides of various transition and rare earth metals. A number of these additives, e.g. SnO2, TiO2, Dy2O3 and Tb4O7, caused little or no difference in the selectivity to C2 products achieved with the resultant catalyst while considerably lowering the temperatures required to give the optimum yields of C2 products. Several other additives caused slight improvements (e.g. La2O3 and NiO) without changing the selectivity and some lowered the temperature for a particular conversion (e.g. CoO, MnO2, PbO and Bi2O3). A number of aspects of some of these catalysts are examined, including the nature of the phases present in the calcined materials, the decomposition of carbonate phases in the catalysts, the effect of promoter concentration and the ageing behaviour under oxidative coupling conditions. A comparison of the various systems shows that the Li/Sn/MgO is an extremely promising catalyst system for the oxidative coupling of methane.