Samples of the isotropic hexahydrate of tricalcium aluminate were dehydrated in a current of dry air at various temperatures up to 1050°. The water retained and the lime liberated were determined. The results of earlier preliminary studies in this laboratory were confirmed. At low temperatures an isotropic hydrate of the composition [Formula: see text], having a refractive index of 1.543 ± 0.003, is formed. This hydrate is stable in dry air up to a temperature of approximately 275° and when once formed at lower temperatures has a very slow rate of decomposition below 350°. On continued dehydration above 275° decomposition occurs with the liberation of calcium oxide. Between 550° and 950° quantitative decomposition occurs into calcium oxide and an aluminate of the composition 12CaO∙7Al2O3. In the upper part of this temperature range prolonged heating causes slow recombination of the solid decomposition products, and above 950° this effect becomes marked. Dehydration of the hexahydrate in a current of dry nitrogen gives similar results. The experiments indicate that the system [Formula: see text] would serve as an efficient drying agent at temperatures below 100°.