The thermal decomposition of ammonium perchlorate in vacuo and under small initial pressures of nitrogen, to suppress sublimation, has been investigated in the temperature ranges 220 to 280 degrees C and 380 to 450 degrees C. The experimental techniques for following the decomposition and subsequent analysis of the products are described and gas analysis results given. In the low-temperature range only 30% decomposition occurred though the 'residue' was still ammonium perchlorate. In vacuo sublimation occurred all the time and also after decomposition had ceased which indicated that the reaction was not in the vapour phase. Some of the properties of the sublimed material and the 'residue' were investigated; in particular, it was found that the residue which was porous in texture (the decomposition had occurred throughout the crystal) could be 'rejuvenated' by exposure to a solvent vapour. The crystal transformation at 240 degrees C from orthorhombic to cubic, the addition of impurities which might be intermediate decomposition products, and the addition of some metallic oxide catalysts, were also investigated.