Hexagonal boron nitride (hBN) and aluminum oxinitride (AlON) composites were synthesized by combustion reaction of powder mixtures of Al–B2O3–AlN systems under a low pressure of nitrogen gas (0.5 MPa). Explosive combustion reaction of Al–B2O3 systems under the same nitrogen pressure produced alumina, aluminum borate, AlN, and AlON depending on the binary mixing ratio, but no trace of BN phases could be identified. Most of the elemental boron product remained unreacted and amorphous. On the other hand, AlN addition as a diluent in the range of 15–30 wt% was effective in producing hBN phase and forming AlON–BN composites. In the composition range of the ternary mixture of Al, B2O3, and AlN, where significant BN formation was identified, the primary role of AlN was to react with B2O3 to produce BN and α-Al2O3. The temperature profile obtained during the combustion reaction by a thermocouple imbedded in the middle of the powder bed revealed that the initial nitridation reaction of aluminum metal provides the heat required for the combustion reaction, creating a state of a “chemical oven.” The reaction product, α-Al2O3, reacted subsequently with AlN to produce AlON phases to give final AlON–BN composites. The combustion reaction was highly unstable and followed a mixed mode with a regularly reversing spinning mode for aluminum nitridation reaction in the surface region and an oscillatory mode for the BN formation reaction in the subsurface region.