To study the ambient analog of the deep mantle MgSiO3 perovskite to post-perovskite phase transition, high-temperature drop calorimetry experiments of perovskite and post-perovskite phases of CaIrO3 system as well as high-pressure phase equilibrium experiments in the CaIrO3 system were made. The enthalpies for dissociation of CaIrO3 (298 K) to CaO + Ir + O2 (1573 K) were 486.7 ± 9.2 kJ/mol for post-perovskite and 454.5 ± 12.5 kJ/mol for perovskite. From the difference between them, the phase transition enthalpy from perovskite to post-perovskite at 298 K is −32.2 ± 15.5 kJ/mol. This gives 2.7 ± 15.6 kJ/mol as formation enthalpy of CaIrO3 perovskite from CaO + IrO2 at 298 K. Using the phase transition enthalpy and volume change of −0.48 ± 0.02 cm3/mol determined in this study, the phase equilibrium boundary is calculated to be P (GPa) = 0.040 T (K) − 67.1. The strongly positive slope agrees with that obtained in high-pressure experiments. This is consistent with a large positive Clapeyron slope of post-perovskite phase transition in MgSiO3 recently reported from experimental and theoretical studies.