Abstract The oxidation behavior of multiphase Mo, Mo 3Si, and Mo 5SiB 2 (T2) alloys was examined at 800 and 1300 °C. The alloys were cast and heat treated at 1600 °C before the cyclic oxidation tests were performed. The compositions of the alloys with Si/B ratios of 0.5, 0.71, and 1.0 were chosen near the L=(Mo)+T2 eutectic trough to produce a fine dispersion of the different phases. The multiphase alloys showed poor oxidation resistance at 800 °C while a protective glass scale formed at 1300 °C. At 800 °C the oxidation resistance improved as the volume fraction of the T2 phase increased. Conversely, at 1300 °C, the glass scaled that formed on the alloys with a Si/B ratio of 0.5 was not protective. However, a protective glass scale did form on the alloys with a higher Si/B ratio. The near eutectic alloy with a Si/B=1 was found to have the best oxidation resistance at 1300 °C. The results indicate the Mo 3Si phase provides a source of silicon for controlling the composition of the glass scale at 1300 °C.