To date, research on the low temperature (<240 degrees C) decomposition of ammonium perchlorate (AP) has typically presented decomposition data either as mass loss curves or Avrami equation fits. Neither approach measures true decomposition reaction rates, which should be a fundamental and early consideration in understanding the chemistry underlying AP decomposition. In addition, concern for low temperature AP decomposition usually occurs in confined environments where unvented product gases may react in a heterogeneous manner with the pristine AP material. In this work, we show evidence that decomposition gas products play a role in promoting the decomposition of solid AP, which suggests that heterogeneous reactions are a major driving force behind these decomposition reactions. A simple heterogeneous chemistry model is proposed to describe the growth and evolution of pores as AP decomposes. The model qualitatively predicts a size dependence on maximum decomposition rates with respect to particle size using kinetic information derived from literature. The experimental data and model provide some insight into decades-old questions surrounding the unique decomposition behavior of AR. An understanding of the role that heterogeneous reactions play in AP decomposition may be used to develop gas scavenging strategies that can protect AP-based propellants from thermal threats. Published by Elsevier Inc. on behalf of The Combustion Institute.