Alloys of Mo-Si-B with different compositions and phase morphologies were cyclically oxidized in dry air at 1200degreesC or isothermally exposed to an H-2-H2S-H2O-Ar environment at 800degreesC Effects of the multiphase nature (composition, morphology) of the Mo-Si-B system on corrosion product formation were evaluated. Preliminary analyses suggested that sulfide formation mimicked the starting alloy microstructure while reactions with oxygen as the sole oxidant reflected cooperative behavior among the different phases. Low corrosion rates under sulfidizing conditions were observed and moderate-to-good high-temperature oxidation resistance was found for certain cases.