We have studied thermodynamic phase stability in the Mg-B system through use of a high-pressure, high-temperature apparatus consisting of a large pressure vessel and an RF induction heater. The incongruent melting temperature for MgB2 was found to be similar to 1450 degrees C at 10 MPa using thermal analysis data applied to both MgB2, powder samples and Mg/B mixtures. The experimental temperature is similar to 300 degrees C lower than temperatures shown in calculated phase diagrams of the Mg-B system at the same pressure and the discrepancy demonstrates the need for further experimental investigations of phase stability in binary Mg-B and ternary Mg-B-X systems. In this study C (as an impurity in boron) was found to have a large effect on the peritectic temperature, with a relatively small (0.7 wt% C) impurity concentration resulting in a similar to 40 degrees C elevation of the peritectic temperature. Along with morphological characterization, XRD and EPMA analyses were used to identify phases and confirm the peritectic transformation in the Mg-B phase diagram. (C) 2013 Elsevier B.V. All rights reserved.