The degradation of HMX-based high explosives (HMX, PBX 9404, and PBX 9501) with sodium hydroxide solutions is described. To obtain practicable reaction rates, the reaction was carried out in a pressurized reactor at temperatures up to about 155 degrees C. Above about 70 degrees C, mass transfer rates significantly affect the observed reaction rate. Therefore, a solid-liquid mass transfer model, based on gas-liquid film theory, was developed to describe the reaction rate. This model successfully predicted the experimentally observed degradation of explosives. Similar work with sodium carbonate solutions was reported previously.(11) Faster reaction rates were observed with sodium hydroxide, a stronger base. Sodium hydroxide is preferred when the explosive contains a base-resistant binder, such as the binder used in PBX 9501, or when large, pressed pieces of explosives are used. Sodium carbonate hydrolysis and sodium hydroxide hydrolysis yielded the same degradation products.