Accurate oxygen-content analysis by means of a coulometric redox titration method specially devised for the purpose shows that as-synthesized (in 5% H 2/Ar) samples of the recently reported novel solid oxide fuel cells anode material Sr 2MgMoO 6− δ contain oxygen vacancies with a concentration of δ≈0.05. Oxygen contents and the resultant Mo-valence values are also analyzed for various A 2MgMoO 6− δ samples in order to reveal both the isovalent and aliovalent A-site cation substitution effects.