Gaseous salts of oxygen-containing acids have recently become the objects of careful research, as the study of these compounds is important to understand the nature of the high-temperature processes in which the gaseous phase participates. The formation of molecules of the type MXO₂ and MXO₃ (M = Ca, Sr, Ba, and X = P, V, Nb, Ta), stable at high temperatures, was investigated by mass spectrometry. For these compounds partial vapour pressures were measured with a reference-ion-current method. The equilibrium constants and reaction enthalpies for the gas-phase reactions that form the ternary compounds, and the standard enthalpies of formation and atomization have been calculated.