The reactions of ten main group acidic oxides and oxyanions with molten alkali metal carbonate eutectics have been studied by thermogravimetry. Products have been identified by IR and Raman spectroscopy and stoichiometries suggested for the several reactions as oxide is progressively incorporated. Ultimately the tetrahedral ortho-oxyanion was formed except with the less acidic oxides (As 2O 3, Sb 2O 3, SiO 2). Sulphite was partially oxidised by the carbon dioxide atmosphere to sulphate.