Abstract The thermal decomposition of ammonium perchlorate (AP) in its cubic modification has been studied in the temperature range 300–390°C. Two distinct regions of temperature dependence are observed for the rate constants of the decomposition. The activation energies in the two regions are found to be 20 ± 2 kcals/mole and 60 ± 2 kcals/mole. Prior mechanical and thermal treatment of the materials is seen to result in a marked increase in the thermal reactivity of AP in the range 300–370°C. Differential thermal analysis of pre-compressed AP shows pronounced changes in the exothermic characteristics of the material. Prior compression of AP in addition enhances the sublimation of the material. The activation energies are not altered by the pretreatment. The results are explained in terms of crystal imperfections and the role of dislocations in the thermal decomposition of AP.