Abstract Single-phase regions in Mo-rich Mo–Si–B alloys at 1800°C were experimentally determined using a field-emission (FE) gun-type electron probe micro-analyzer (EPMA) in this study. A quantitative analysis by FE-EPMA was conducted with the calibration method we applied in our previous study to improve the accuracy in the B measurements. The compositional ranges of the determined Mo solid solution, silicide and boride phases were in good agreement with those of Mo–Si and Mo–B binary phase diagrams. On the other hand, the determined solubility of B in Mo solid solution, Mo3Si and T1 (Mo5Si3) were quite different from that indicated in previously reported ternary phase diagrams. The compositional region of the T2 (Mo5SiB2) single-phase ranges from 9.7 to 13.3at% for Si and from 23.5 to 26.8at% for B. The following two points were made clear by the ternary phase diagram as determined in this study. One is that no stoichiometric T2 composition is in the T2 single-phase region, confirming no T2 single-phase material can be obtained at the stoichiometric composition at 1800°C. The another is that the T2 region of this study expands to a Si-rich area from the stoichiometric composition.