Four different Mo-Si-B alloys consisting of Mo, Mo5SiB2, and Mo3Si were produced by hot pressing different Mo5Si3, Mo, B powder blends at 1600degreesC. The volume fraction of the metal phase was varied between 20 and 78 vol.%. The consequences of changes in phase fractions and microstructure are discussed in terms of the room temperature fracture toughness and the oxidation resistance at 1300degreesC.