The thermal degradation of H6TeO6in air has been evaluated critically. Evidence is presented for a decomposition mechanism involving step-wise dehydration of H6TeO6via non-stoichiometric amorphous solids to polymetatelluric acid and up to a composition corresponding to pyrotelluric acid. No morphological changes were observed during these structural variations and no evidence was found for the formation of allotelluric acid. Further dehydration is accompanied by reduction, which, depending upon the experimental conditions accounts for the considerable variety of results reported previously. Crystalline Te(VI)-Te(IV) oxides are obtained at about 550‡ from which TeO2is formed by additional calcination at about 620