The thermal decomposition of the peroxides, triacetone triperoxide (TATP) and hexamethylene triperoxide diamine (HMTD) was investigated as a function of pressure, temperature and atmosphere by T-jump/FT-IR spectroscopy. In an Ar atmosphere at fast heating rates, the dominant decomposition product of TATP was methane rather than acetone which dominates at slow heating rates. The products of TATP were different in Ar and in air which implies that decomposition occurs primarily in the gas phase. In contrast, the products from HMTD were independent of the surrounding atmosphere implying that it decomposes primarily in the condensed phase. The IR spectra of the decomposition products of HMTD exhibited some dependence on pressure but not on the pyrolysis filament temperature.