A model has been derived for the subsolidus and melting relationships in the system MgO-H2O-CO2 for pressures up to 100 kbar. Using both low- and high-pressure experimental data, the ΔHO and ΔSO for the subsolidus reactions Mg(OH)2 → MgO + H2O, MgCO3 → MgO + CO2, and MgCO3 + 4H2O → Mg(OH)2 + CO2 are estimated to be respectively: ΔH0 = 18400 cal/mol and ΔSO = 33.16 cal/°mol, ΔH0 = 20140 cal/mol and ΔSO = 34.87 cal/°mol and ΔHO - 1740 cal/mol and ΔSO =1.71 cal/°mol. In the presence of MgCO3, vapor is buffered to high values of H2O/CO2 except for a narrow pressure interval just above the reaction MgCO3 → MgO + CO2. As pressure is increased the vapor coexisting with an MgCO3-bearing assemblage becomes richer in H2O. MgCO3 first melts at an invariant point located at 23 kbar and 155O°C. Mg(OH)2 melts at an invariant point estimated to be near 58 kbar and 131O°C. Mg(OH)2 + MgCO3 + vapor melt together at a eutectic at pressures greater than or equal to that of an invariant point located near 46 kbar and 121O°C. The composition of the eutectic liquid on the join Mg(OH)2-MgCO3 is estimated to be 73 mole percent Mg(OH)2 plus 27 mole percent MgCO3. The system MgO-H2O-CO2 provides the thermodynamic basis for estimating the distribution of H2O and CO2 between coexisting liquids and vapors in the presence of hydrates and carbonates, and is an essential first step leading to the interpretation of similar phase relationships in the model mantle system CaO-MgO-Al2O3-SiO2-H2O-CO2.