The single -MoSi2 phase was prepared by the Na flux method and its stability against thermal, oxidative, and mechanical treatments was investigated. The X-ray diffraction results show that the single phase is formed at 600 A degrees C within 1 h using pre-mixed Mo and Si powders with a Si/Mo molar ratio of 2.00-2.25. By energy-dispersive X-ray spectroscopy, the produced powder is found to be Si-deficient with a Si/Mo molar ratio of 1.87-1.96. The differential thermal analysis shows that the phase transforms into the -MoSi2 phase at 815 A degrees C at 10 K/min with the segregation of a small amount of Mo5Si3. The transformation heat is -5.5 kJ/mol and the activation energy calculated by the Kissinger method is 290 kJ/mol. Thermogravimetry reveals that the -MoSi2-x powder oxidizes significantly at 400-600 A degrees C via the pest oxidation mechanism while it is resistant to oxidation at 700 A degrees C for 5 h similarly to the -MoSi2 phase. At last, mechanical milling on the -MoSi2-x powder with a planetary ball mill up to 216 h demonstrates that this powder is stable under a severe mechanical treatment.