The system MgO-CO2-H2O has been studied up to 1,400° C and 4,000 bars pressure using the sealed-capsule quenching technique. No melting was observed. At 1,000 bars pressure magnesite dissociates at 780° C, and brucite at 635° C, to periclase and vapor. The univariant reaction MgCO3÷Mg(OH)2 ⇌ MgO + V proceeds at 630° C, at 1,000 bars and at 700° C, at 4,000 bars. Solubility measurements show that, at 1,000 bars and temperatures up to 1,000° C, less than 1.5 weight per cent MgO is dissolved in the vapor phase. Brucite is unstable in the presence of vapors containing more than a small amount of CO2. The maximum percentage of CO2 in a vapor that can coexist with brucite increases with decreasing pressure and with increasing temperature: 6 weight per cent CO2 is the maximum at 630° C, 1,000 bars, and 4 weight per cent at 700° C, 4,000 bars. The phase relations in the isobaric TX prism for 1,000 bars pressure are described. The results illustrate two dissociation reactions, decarbonation and dehydration, occurring in the presence of a vapor phase containing two volatile components, H2O and CO2. Applications to metamorphism are briefly discussed.