CaO was impregnated with different sodium compounds (NaNO3, NaOH, NaBH4, NaNH2, Na2CO3), dried and calcined (873-1023 K). For these materials, which were used as catalysts in oxidative methane coupling (OCM), surface compositions were determined by X-ray photoelectron spectroscopy after drying, calcining and exposure to OCM. Surface composition depends on the type of precursor used, its bulk amount and calcination temperature. After exposure to OCM surface sodium content decreases. C-2+ selectivity is markedly affected by the sodium content on the surface, a minimum content of approximately 30 at.-% of sodium is required for maximum selectivity.