This paper presents experimental results regarding the sources that influence the strength, shape, and decay rate of a blast wave produced by the high-speed impact of Al projectiles with inert aluminum oxide targets at speeds between 1.1 and 1.3 km/s. The impacts were conducted in oxidizing and non-oxidizing atmospheres to isolate the effect of chemical energy release on the blast wave. Results show that the shape and strength of the blast wave are determined by the energy transfer processes associated with the impact kinetic energy and metal reaction during the first few microseconds following impact. With oxygen-enriched atmospheres, a threshold oxygen concentration exists between 30 and 40%, at which the blast Mach number, and hence peak blast overpressure, is significantly increased. The decay rate of the peak blast overpressure is reduced, particularly in Ar–O2 40%, suggesting that the irregular shape of the blast wave and particle combustion over 100  µs can influence the decay rate.