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The composition and phase constituency of Mo–Si–B alloys are known to be important parameters in determining the oxidation response. For three phase Mo+T₂+Mo₃Si alloys with constant composition and phase constituency, it is observed that a refined microstructure scale provides superior oxidation resistance. The transient stage of oxidation is shortened and the recession of the alloy is decreased with microstructural refinement. In order to identify the phase interaction during the transient stage, oxidation of each of the three alloy phases, Mo, Mo₃Si (A15) and Mo₅SiB₂ (T₂) has been investigated separately. Quantification of the separate phase size distributions by image analysis was coupled with the individual phase oxidation response to evaluate the overall oxidation behavior and phase interaction effects. A kinetic model for oxidation of Mo–Si–B alloys is proposed that incorporates the key role of …