(1) It was found that the vapour pressures of decomposition of diaspore could be calculated by the following equation in the range from 400° to 500°. Logπ=14.820–9255.1/T. (2) In the case of bauxite, the curve of decomposition pressure was found to be consisted of two parts. The first part can be expressed by the equation logπ=6.5277–2206.5/T, in the range from 200° to 340°, and the second roughly by logπ=5.2566–1780/T in the range from 350° to 450°. (3) The heats of decomposition of diaspore and bauxite were calculated by the integrated equation of Clapeyron and Clausius. (4) The specific heat of diaspore was determined by the calorimetric investigation and found to be c=0.2085+0.000127 t.