The response of propellants and explosives to thermal stimuli is an important aspect of their behavior, especially for their safe storage and handling. In the case of ammonium perchlorate (AP), the principal oxidizer used in solid composite propellants, exposure to elevated temperatures causes various levels of decomposition and morphological changes. These changes occur in both neat AP particles and AP based propellants. The hazards associated with thermally damaged, or partially decomposed AP are investigated. It is found that AP is more thermally stable after partial low temperature decomposition. Also, it is found that the extent of decomposition of the AP particles is strongly influenced by prior thermal damage. This dependency is attributed to the exhaustion of nucleation sites on certain crystal planes. Specific surface area measurements of the thermally damaged particles show that the particles recrystallize before they can decompose to a further extent in low temperature decomposition. The behavior of partially decomposed AP in the presence of a propellant binder is also examined.