The temperature histories of aminoguanidinium 5,5'-azobis-1H-tetrazolate (C(4)H(14)N(18), AGAT) were measured in order to construct a thermal decomposition model of the compound. ne effects of chamber pressure and AGAT particle size were also examined. The results of the study suggest that the thermal decomposition of AGAT occurs in three phases: solid phase, condensed phase, and residue. It was found that the condensed phase consists of decomposition zone I where dramatic temperature rise occurs, decomposition zone II where gradual temperature rise occurs, and the cooling zone where decomposition and temperature stop rising. It was also suggested that the temperature of the decomposition surface which is the interface of the solid phase and the condensed phase was similar to 500 K, and that N(2) and NH(3) were suggested to occur in the vicinity of the decomposition surface of decomposition zone I. In addition, it was suggested that the thickness of the decomposition zones I and II decreases and that the maximum-temperature-reached increases with an increase in atmospheric pressure. ne rate of decomposition of AGAT was found to follow Vieille's equation and the rate of decomposition increases with an increase in pressure. The rate of decomposition increased slightly with an increase in particle size.