The thermal stability of hexanitrostilbene (HNS) after exposure to 20, 150, 175, 200, and 2250/C can be inferred from precise measurements of the detonation velocity of a 2-grain-per-foot, aluminum-sheathed mild detonating fuze (MDF). The results indicate that the thermal stability of small diameter charges of HNS is greatly reduced by exposures above 2000/C. The method used to prepare the bulk HNS powder has a decided effect on the thermal stability of the resulting MDF. The purity of currently available HNS is the limiting factor to continued investigation of the thermal stability of HNS.