The formation of associates in the vapor phase over a mixed oxide is analyzed in relation to the difference in acid-base properties and volatility between its constituent oxides. A method is proposed for assessing the standard enthalpy of formation of gaseous oxy salts. Experimentally determined standard enthalpies of formation are presented for gaseous salts formed by CrO, CrO ₂, MnO, FeO, CoO, NiO, and MgO.