Supercritical and vapor-phase reactions of oxidizers and fuels under pressure are of commercial interest. Because mixtures of propane and nitrous oxide have the potential to be explosive, a model mixture was studied with the intent of defining safe operating boundaries. This work investigated the nitrous oxide/propane oxidant/fuel system, diluted with the inert solvent carbon dioxide, and a catalyst as an accelerant, to determine the safe operating regions for the oxidation reaction. Using an ARC® calorimeter for the evaluations, variables included initial system density, catalyst, and mixture compositions. Density of the system ranged from 0.05 to 0.62 g/cm3. A number of catalysts were investigated with 10% Pd on carbon being the most active. Strongest reactivity was observed when the mixture was stoichiometric to the complete combustion products of nitrogen, carbon dioxide, and water. Three distinct regions of reactivity were observed: mild reactivity (starting at the sensitivity of the ARC of dT/dt = 0.02 to about 0.1° C/min); strong exothermic reaction (dT/dt ≅ 1° C/min); and flammable and/or explosive (a distinct and substantial jump in dP/dt in <1 s). As a result, a boundary has been proposed for safe mixtures of nitrous oxide, propane, and carbon dioxide, and a basis for determining safe operating compositions for other systems. The computed adiabatic reaction temperature was used to correlate the experimental results to establish the boundary for safe operation. © 2005 American Institute of Chemical Engineers Process Saf Prog, 2005