The equations of state of forsterite, wadsleyite, ringwoodite, MgSiO3-perovskite, akimotoite, and postperovskite are set up by joint analysis of experimentally measured isobaric heat capacity, bulk moduli, thermal expansion depending on temperature at ambient pressure, and volume at room and higher temperatures. Modified equations of state based on the Helmholtz free energy are used to construct a thermodynamic model. The derived equations of state permit calculation of all thermodynamic functions for the minerals depending on temperature and volume or temperature and pressure. A phase diagram of the system MgSiO3–MgO is constructed based on the Gibbs energy calibrated using the referred experimental points. The seismic boundaries at depths of 410 and 520 km and in the zone D’ are interpreted on the basis of the phase transitions. The global upper/lower mantle discontinuity at a depth of 660 km remains debatable; it is in poor agreement with experimental and computational data on the dissociation of ringwoodite to perovskite and periclase.