This paper focuses on the relationship between the topology as well as scale of the microstructure of Mo-Mo(sub 3)Si-Mo(sub 5)SiB(sub 2) alloys, and their creep strength and fracture toughness. For example, the creep strength of Mo-Si-B alloys is improved by reducing the (alpha)-Mo volume fraction and by making the (alpha)-Mo phase discontinuous. The fracture toughness is improved by increasing the (alpha)-Mo volume fraction and by making the (alpha)-Mo phase continuous. Room temperature stress intensity factors as high as 21 MPa m(sup 1/2) were obtained. The room temperature fracture toughness of Mo-Si-B alloys can also be improved by microalloying with Zr. The room temperature ductility of Mo itself can be improved by adding MgAl(sub 2)O(sub 4) spinel particles suggesting yet another way to improve the ductile phase toughening of Mo-Si-B alloys.