Photomicrography and software-based quantitative image analysis were used to examine microstructural morphology of samples of a plastic bonded explosive, PBX 9501, which had been thermally damaged under varied confinement conditions. Samples were damaged to an advanced state, as would be attained just prior to cook-off. We observed significant changes in morphology, when compared to pristine material, which contribute to higher gas permeability. Changes included the formation of channels and pores, as well as fracturing of the HMX crystalline component. We discuss both qualitative and quantitative observations of morphology and whether they are likely to affect sensitivity and behavior of PBX 9501.