Abstract The microstructure and phase constitution of Mo(Si1−x,Alx)2 alloys (x=0.03, 0.1 and 0.4) prepared by self-propagation high-temperature synthesis is first investigated using SEM, EDS and XRD analysis. Then the lattice parameters and adiabatic temperature are calculated. Based on the above experimental and calculated results, the variation mechanism of diffraction peaks and phase transformation subsequence of the Mo–MoO3–Si–Al powders is discussed. Results show that, when the self-propagation reaction is over, there are a homogeneous Mo–Si–Al alloy melt and a fused Al2O3 with lower density at top. Subsequently, MoSi2 or Mo(Si,Al)2 phase nucleates and grows as a primary phase in the Mo–Si–Al alloy melts, and then Al, Si substances are generated from the intergranular residual Al–Si liquid according to Al–Si binary phase diagram. The Al increase in the starting powder mixtures leads to the Al concentration increase in the Mo–Si–Al alloy melt. Consequently, MoSi2 is transformed to Mo(Si,Al)2 to phase in which Si is replaced by Al atoms and Al substance in the intergranular zones increased accordingly.