The thermal decomposition of neat hexamethylene triperoxide diamine (HMTD) was examined over the temperature range 100 and 180 degreesC. It was found to be first-order up to 150 degreesC and produce similar to2 mole of gas/mole HMTD; primarily CO2 but also trimethylamine and ammonia. Above 150 degreesC, the decomposition became nearly instantaneous and gas production more than doubled. Instead of carbon dioxide, CO was the main product (about 3 mole/mole HMTD) and under air no trimethylamine was observed from the thermolysis, The observed changes in products may indicate a change in mechanism, but they can also be explained by a secondary reaction, the oxidation of trimethylamine to HCN and methanol. The latter interpretation is based on the fact that when available oxygen was limited, i.e. when the decomposition was performed under vacuum, some trimethylamine was observed in the high temperature thermolysis. Two possible mechanisms are suggested, one free radical and the other ionic.