A bomb calorimeter for measuring the heat of detonation of 25 g charges of high explosive is described. A complete calorimetric measurement can be made in 1 h with a precision of 0.3%. Calorimetric measurements and analysis for PETN are described. The data are interpreted with the aid of thermodynamic and hydrodynamic computer calculations. For unconfined or lightly confined charges, the released energy is largely retained in the products which are shocked considerably off the Chapman‐Jouguet isentrope by reflections from the bomb wall. For heavily confined charges, the energy is largely converted to kinetic and internal energy of the confining case, and the essentially unshocked products expand along the Chapman‐Jouguet isentrope. The products of detonation are found to freeze out at 1500 to 1800°K. The heat of detonation of PETN at 298°K is 1490±6 cal/g, H2O(l).