The anhydrous products resulting from the dehydration of Zr(SO4)2,4H2O in external atmospheres of controlled H2SO4 and SO3 partial pressures consist of three polymorphs, two of which, γ- and β-Zr(SO4)2, are metastable and equilibrate to the stable phase, α-Zr(SO4)2. The rates of decomposition of these anhydrous sulphates to cubic ZrO2 have been measured and activation energies for the processes calculated. From these data the vapour pressure-temperature diagram for the anhydrous region of the ZrO2-SO3-H2O system has been delineated. This shows the existence range of γ-Zr-(SO4)2 lies within that of β-Zr(SO4)2 while both lie within that of the stable α-phase. The function of physical factors, such as particle and sample size and heating technique, in controlling phase formation during dehydration in air is interpreted in terms of the vapour pressure-temperature diagram. © 1969, CSIRO. All rights reserved.