Enthalpy for MgSiO3 orthopyroxene--perovskite transition at 298 K was measured by differential drop-solution calorimetry. Based on four measurements of MgSiO3 perovskite, the ΔH¿298 of 102.2¿2.5 kJ/mol was more precisely determined than that in the previous study. The enthalpies of transitions from MgSiO3 ilmenite to perovskite and from MgSiO4 spinel to perovskite + periclase were calculated to be 43.2¿5.0 and 86.1¿3.6 kJ/mol, respectively. The boundaries of these transitions were calculated using the above data together with physical properties of relevant phases, including recently determined properties of perovskite: heat capacity, entropy, and temperature dependence of bulk modulus. The calculated P--T slopes for the ilmenite-perovskite transition and the spinel dissociation are -3.4¿2 and -3¿1 MPa/K, respectively. The latter slope may suggest that the upper and lower mantle convect separately but intermittent mixing occurs between them. ¿ American Geophysical Union 1993