For the oxidative coupling of methane a relationship between the state of surface oxygen and C-2 selectivity of Nd2O3 and SrO (1 at%)/Nd2O3 catalysts was derived from contact potential differences (CPD) between a reference electrode and the catalyst surface as a function of oxygen partial pressure and temperature. The influence of the concentration of oxygen anion vacancies of the oxide lattice on the state of surface oxygen was elucidated by measurements of the electrical conductivity and of isotopic oxygen exchange on the undoped and SrO-doped Nd2O3, Oxygen defects facilitate the oxygen incorporation into the lattice of SrO(1 at%)/Nd2O3 as indicated by the lower CPD changes due to oxygen adsorption and the higher extent of oxygen exchange for SrO-doped Nd2O3 compared with pure Nd2O3 Thus, the C2+ selectivity of a SrO(1 at%)/Nd2O3 catalyst was enhanced since the concentration of weakly adsorbed (molecular) oxygen species leading to deep oxidation of hydrocarbons is lower on a defect rich solid surface.