Enthalpy of the MgSiO3 pyroxene-tetragonal garnet transition was measured by high-temperature solution calorimetry, giving ΔH9830=30.80¿3.11 kJ/mol at 983 K. Heat capacities of MgSiO3 pyroxene and tetragonal garnet were also measured by differential scanning calorimetry from temperatures of 150 to 700 K. Using these calorimetric data and published thermodynamic data, equilibrium phase boundaries of high-pressure transitions in MgSiO3 were calculated at pressures up to 26 GPa and at temperatures up to 2600 K. In calculating the stability field of MgSiO3 tetragonal garnet, the effect of partial disorder of Mg and Si cations in the octahedral sites of garnet on the phase boundaries was examined. The calculated stability field of tetragonal garnet was generally consistent with the high-pressure experimental data. The calculated boundaries for the pyroxene-garnet and garnet-perovskite transitions have negative and positive slopes, respectively. ¿ American Geophysical Union 1993