Adiabatic Calorimetry is a suitable technique for characterising highly exothermic processes, such as decomposition reactions. Closed-Cell Adiabatic Calorimetry (CCAC) has been used for years as a reference technique for thermal hazard assessment of pure chemicals and mixtures. In this paper some of the results of a comparison study involving two of the most popular adiabatic calorimeters, ARC and PHI-TEC II, are presented. This project was carried out during 1994 and 1995 at the Stazione Sperimentale per i Combustibili (Italy) and at the Chemical Engineering Department of the Institut Quimic de Sarria (Spain).