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A MoSi 2 coating was successfully formed on a Mo-9Si-18B alloy, consisting of Mo 5 SiB 2 (T 2 ) and Mo solid solution (Mo ss ) phases, using pack cementation with Si. Isothermal and cyclic oxidation tests of pack-cemented Mo-9Si-18B alloys were performed at 1300 °C and 1500 °C. Steady-state oxidation rates at both temperatures are almost equal to those of pure MoSi 2 . The MoSi 2 layer is completely transformed into Mo 5 Si 3 (T 1 ) containing B after oxidation at 1500 °C for 24 hours. Thermal expansion of the T 1 phase is anisotropic, but a [001] texture in the growth direction for the columnar grains in the T 1 layer reduces thermal stresses generated around the phases. Evolution of T 1 layers during oxidation between 1300 °C and 1500 °C was investigated; their growth rate constants and the interdiffusion coefficient of Mo and Si in the Mo-Si-B system have been evaluated and compared with those in the binary Mo-Si system. Furthermore, we have studied phase transformations in a simpler system MoSi 2 vs T 2 using MoSi 2 /T 2 diffusion couples. Layers of T 1 and MoB ϩ T 1 were formed in the diffusion zone during oxidation at temperatures between 1400 °C and 1600 °C. This behavior is different from that of the pack-cemented Mo-9Si-18B alloy. Pack-cemented T 2 single crystals show a diffusion structure similar to that of MoSi 2 /T 2 diffusion couples, but the ratio of layer thickness is different. Based on these diffusion results, a method for extending the lifetime of the MoSi 2 layer is proposed.