Based on experimental data for reactions involving sapphirine in the system MgO–Al2O3–SiO2 (MAS), thermodynamic properties of the sapphirine solid solution end-members have been optimized for both ideal and non-ideal models, and internally consistent P–T grids are proposed. According to the calculated P–T relationships in MAS, the sapphirine + quartz assemblage, which is widely recognized as indicative of ultrahigh-temperature metamorphism, can be stable down to 840 ° C and 0.67 GPa, i.e., at temperatures up to 150 ° C lower than estimated by Kelsey et al. (2004). The sapphirine + kyanite assemblage has been found stable at temperatures below 860 ° C and 1.13 GPa, whereas the sapphirine + forsterite assemblage may be stable below 800 ° C only under specific conditions of a very low activity of water. Calculations of P–T relationships involving sapphirine using the Thermocalc dataset revealed its inconsistency with both experimental and natural assemblages.