The evaporation of salts of d-elements (manganese molybdates and tungstates) was investigated by the method of high-temperature mass spectrometry at the energy of ionizing electrons of 25 eV. The standard enthalpies of formation of manganese molybdates and tungstates were first obtained by measuring equilibrium constants and enthalpies of gas-phase reactions of salt decomposition into oxides. The results of quantum chemical calculations of geometric and electronic structure and vibration spectra of MnXO₄ (X = W, Mo) molecules using the program complex GAMESS are presented.