Allopurinol sublimates when subjected to thermogravimetry in a nitrogen atmosphere. The process is zero order and plots of in k venus reciprocal temperature (K) give a value of 132.64 kJ mol(-1) for the activation energy. On the basis that equilibrium is achieved at all temperatures the data is used to establish a plot of the vapor pressure curve versus temperature from which a Clausius-Clapeyron relationship can be used to establish the DeltaH for sublimation. The method requires that the equipment is calibrated and data for p-aminobenzoic acid is used for this purpose.