Basic sites of various oxide catalysts for oxidative methane coupling have been studied with the IR spectra of CO2 probe molecules. It is possible to determine the concentration and strength of surface basic sites, by studying carbonate compounds, which form on surface oxygen sites. Since the surface structure is non-uniform, its oxygen ions carry different negative effective charges and thus produce carbonates, exhibiting different IR spectra. A correlation between the site strengths and carbonates’ spectral behavior, found earlier, allows us to distinguish the sites rather well. The catalytic activity of oxide catalysts was found to depend on the presence and concentration of the strongest basic sites on the oxide surface.