Solutions of nitramine and nitrate ester explosives and model compounds were thermolyzed at various hydrostatic pressures and their rates of decomposition were measured. The effects of pressure on their rates were used to infer the mechanism of their initial decomposition steps. Most nitramines, including the explosive octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), appear to undergo homolysis of the N-NO2 bond, because their reaction rates decrease with increasing pressure. Exceptions are hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and the model compound, 6-nitro-1,2-dinitroso-1,2,3,4-tetrahydroquinoxaline, which react faster with increasing pressure. These two compounds can aromatize by elimination of HNO2 and HNO, respectively. Secondary nitrate esters shift their major decomposition pathway from homolysis of the O-NO2 bond to elimination of HNO3 in the pressure range of 0.4 to 0.8 GPa. The elimination reaction resembles carboxylate ester pyrolysis with E1 character.