The effect of aluminum addition on the structural and low-temperature oxidation behavior of commercially available MoSi 2 was investigated by means of X-ray diffraction and thermogravimetric analysis. MoSi 2 ? x Al compounds (with x = 0.0, 0.10, 0.20, 0.30, and 0.40) were mechanically processed for 6 h starting from a-MoSi 2 and Al powders. It is shown that Al can substitute Si atoms in pre-formed MoSi 2 leading to the formation of b-MoSi 2 (the high temperature phase). The unit cell volume of b-MoSi 2 increases linearly with the Al content. For x \ 0.20, a clear onset temperature for the oxidation of MoSi 2 is discernable at ca. 420°C, while it is barely noticeable for x C 0.20. The normalized mass gain decreases from *12 % m -2 to less than 2 % m -2 when the Al content varies from 0.0 to 0.2. For x C 0.20, the normalized mass gain stays constant at less than 2 % m -2 .