High performance solid rocket motors typically contain ammonium perchlorate (AP) particles as the oxidizer. Ammonium perchlorate provides good performance, but thermal decomposition leads to safety concerns for handling and storing solid propellant. Computed tomography is shown to allow for visualization of the AP decomposition process, providing in-situ, quantitative data. The current work demonstrates the use of nano-computed tomography (nano-CT) scanning to elucidate aspects of AP decomposition by studying partially decomposed 400m diameter AP particles after isothermal heating at 200 degrees C. Data provides insight into shape and location of the pores. Analysis shows that the porosity developed within the particle begins approximately 15m below the particle surface and moves inward as well as outward towards the surface as the heating time increases. No substantial heating time dependence was observed on the pore size distribution. The size and location distributions of decomposition sites forming below the AP particle surface were quantified for the first time. Comparisons to previous work are discussed.