We identify nitryl perchlorate as the essential intermediate in the low temperature thermal decomposition of ammonium perchlorate AP. Evidence supporting this identification includes the analytical detection of an oxidized nitrogenous species in partly reacted AP and the ability of ammonium nitrate and several other nitrates to markedly reduce the induction period to decomposition of AP and to accelerate the subsequent reaction. It is also shown that the measured rate of the reaction of pure AP is in very satisfactory agreement with that estimated to result from this amount of NO
2
ClO
4
present. This mechanism differs from those currently accepted, in which the controlling process is believed to involve the transfer of either a proton or an electron. Our proposal is based on the known instability of NO
2
ClO
4
at reaction temperature (
ca
. 500 K), the enhanced reactivity compared to the ionic alkali perchlorates being ascribed to covalent bond formation O
2
NO─ClO
3
. Subsequent reactions of the products of breakdown of this species, NO
+
, ClO
3
-
and 2O or O
2
, are regarded as capable of oxidizing reactant NH
4
+
(→NO
2
+
), thus regenerating the intermediate. Localized reaction in migrating ‘particles’ of fluid NO
2
ClO
4
, advancing through the reactant and leaving a residue of porous NH
4
ClO
4
, explains the unusual, incomplete low temperature decomposition that is characteristic of AP. The article reports comparative kinetic data for the decomposition of pure AP and the reaction initiated by various added nitrates. Rate studies are complemented by scanning electron microscope examinations of the geometry of interface development and the structure of the decomposed salt. From these and analytical results the role of nitryl perchlorate in AP decomposition is discussed.
