Specific mixtures of ammonium perchlorate (AP) and magnesium are used in fireworks and to demonstrate the strobe effect. The composition easily undergoes deterioration in the presence of moisture, but the effects of this deterioration are complex and not well-understood. This study aims to determine the effects of moisture-induced deterioration products on an AP/Mg mixture. The mixture was subjected to an accelerated aging test with moisture and by mixing the AP with deterioration products. Analysis of the thermal behavior of the samples revealed that the aging test decreased the heat generation and increased the onset temperature of the exothermic peaks. The addition of the deterioration product, magnesium hydroxide, had the same effect on AP. The effects of the material of vessels are slight. The onset temperature of the exothermic peaks for the AP/Mg mixture was slightly changed in the samples stored for durations ranging from 24-144 h, unlike the heat generation. Analysis of the Mg surface in the aged samples indicates that the products coated the Mg powders after the initial stage of deterioration. The thermal stability of AP is observed to be affected by the materials on the Mg surface, and hence, the initial stage of deterioration is effective for thermal stability.