An improved thermally stable, storage stable gas generating composition characterized by both an increased burning rate and a decreased pressure exponent for producing a clean, nontoxic, substantially ash- and solids-free gas for inflating inflatable structures such as vehicle airbags is provided. Optimum combustion efficiencies and operating pressures are achieved in the improved gas generating composition by adding an effective amount of a copper or copper compound catalyst to a gas generating composition that is preferably a eutectic mixture or solid solution of ammonium nitrate, at least one highly oxygenated fuel, potassium or cesium perchlorate or nitrate and an optional binder.