Previous studies in this laboratory and the examination of literature activity data suggest that only the highly basic oxides are effective catalysts for oxidative coupling of methane to form C2+, hydrocarbons. On the considerations of basicity and thermal stability, SrO and BaO are thus expected to have the best catalytic potential. One problem is that in the presence of carbon dioxide, these oxides easily form weakly basic carbonates with high thermal stabilities. Development of effective catalysts was, therefore, thought to involve suitable additives which can provide a mechanism to destabilise the carbonates under reaction conditions. Cations which possess high polarizability and form oxides with low basicity and carbonates with low thermal stability, were considered effective as additive. This paper examines the activity of SrO and BaO catalysts containing MgO, Al2O3 or SiO2 as additives. The activity data show that the moderately basic MgO as additive brings about significant performance improvement in both SrO and BaO catalysts and that the optimum MgO level is about 80 mol-%. In the optimum composition SrCO3 and BaCO3 decompose to oxide at considerably lower temperatures and produce large populations of surface O2- sites responsible for catalytic activity. The performance of the optimum Sr-Mg and Ba-Mg catalysts were further enhanced by implanting impurity levels of Li2CO3, Na2CO3, La2O3, ZrCl4 and other species. In contrast, the acidic SiO2 as additive deactivates the catalysts, whilst weakly basic Al2O3 is beneficial only to barium catalysts, especially at Ba:Al ratios of about 1:1.