An adiabatic vacuum calorimeter of the copper-block type, for measuring enthalpies in the region 350–1800°K, is described. The quantities of heat measured by means of a platinum resistance thermometer located on the central axis of the calorimeter, and a thermometer of the Maier type wound on its outer surface, are identical within the limits of experimental error. The enthalpy of the Calorimetry Conference sample of α-alumina has been determined in the temperature range 400–1250°K, using adiabatic and non-adiabatic techniques. Both sets of data are in excellent agreement with the values computed from the National Bureau of Standards “corrected” equation, the mean deviations being ± 0.07 % and ± 0.09 % respectively.