The vapour pressures of some inorganic sulphates at high temperatures were determined by the combined use of the Knudsen effusion, transpiration, and matrix isolation methods. After a detailed comparison with the results of other investigators, it is concluded that the principal vapour species in the case of K2SO4, Rb2SO4, and Cs2SO4 are the undecomposed sulphate molecules themselves, and in the case of Li2SO4 the decomposition products, Li, SO2, and O2. The sodium salt also decomposes to some extent into Na, SO2, and O2, but it is deduced that the vapour concentration of the species Na2O4 is probably greater than was formerly supposed. The decomposition of alkaline earth sulphates is also discussed, especially in the light of the dependence of the Knudsen effusion results upon the size of the orifice used. On the basis of the vapour constitutions deduced, thermodynamic functions for the important species present are tabulated up to 1400 K or above. Reference is also made to the function of sodium sulphate in the glass-making industry and to the possible mechanism of its corrosive action on furnace walls.