The standard Gibbs energies of formation of RuO2 and OsO2 at high temperature have been determined with high precision, using a novel apparatus that incorporates a buffer electrode between the reference and working electrodes. The buffer electrode absorbs the electrochemical flux of oxygen through the solid electrolyte from the electrode with higher oxygen chemical potential to the electrode with lower oxygen potential. The buffer electrode prevents polarization of the measuring electrode and ensures accurate data. The standard Gibbs energies of formation (ΔfG°) of RuO2, in the temperature range of 900–1500 K, and OsO2, in the range of 900–1200 K, can be represented by the equations
where the temperature T is given in Kelvin and the deviation of the measurement is ±80 J/mol. The high-temperature heat capacities of RuO2 and OsO2 are measured using differential scanning calorimetry. The information for both the low- and high-temperature heat capacity of RuO2 is coupled with the ΔfG° data obtained in this study to evaluate the standard enthalpy of formation of RuO2 at 298.15 K (ΔfH°298.15K). The low-temperature heat capacity of OsO2 has not been measured; therefore, the standard enthalpy and entropy of formation of OsO2 at 298.15 K (ΔfH°298.15K and S°298.15K, respectively) are derived simultaneously through an optimization procedure from the high-temperature heat capacity and the Gibbs energy of formation. Both ΔfH°298.15K and S°298.15K are treated as variables in the optimization routine. For RuO2, the standard enthalpy of formation at 298.15 K is ΔfH°298.15K(RuO2) =−313.52 ± 0.08 kJ/mol, and that for OsO2 is ΔfH°298.15K(OsO2) =−295.96 ± 0.08 kJ/mol. The standard entropy of OsO2 at 298.15 K that has been obtained from the optimization is given as S°298.15K(OsO2) = 49.8 ± 0.2 J·(mol·K)−1.