The pyrolysis of a 50150 mixture of RDX/GAP-diol cured with the isocyanate compound N-100, and a 70/21/9 mixture of RDX/BTTN/GAP-polyol cured with HMDI were studied by the T-jump/FTIR spectroscopy technique. To understand the behaviors better, pyrolysis was also conducted on pure N-100, pure BTTN, GAP-diol cured with N-100, and 70/30 BTTN/GAP-polyol cured with HMDI. Pure N-100 and the cured propellants liberate a large quantity of HMDI upon pyrolysis. This result reveals that the urethane bonds break early in the reaction sequence and the curing agent begins to vaporize from the matrix. The 50150 RDX/GAP mixture decomposed with relatively little smoke or residue, which sharply contrasts with its reported behavior upon combustion. The difference can be attributed to the mismatch between the heating rates of flash pyrolysis and combustion. During pyrolysis the RDX and GAP are able to remain in contact longer such that fuel-oxidizer reactions occur between them. During combustion the two compounds may segregate due to evaporation of most of the RDX Hence the GAP decomposes in the condensed phase giving smoke and residue without the benefit of oxidation by NO2 from the RDX This discrepancy does not exist with the more fuel-oxidizer balanced 70/21/9 mixture of RDX/BTTN/GAP, where the interaction of the fuel and oxidizer species is strong.