The standard Gibbs energies of formation of IrO2, CaIrO3, Ca2IrO4 and Ca4IrO6 have been measured in the temperature range 940-1350 K using solid-state cells with (Y2O3) ZrO2 as the electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode. For the design of appropriate working electrodes, phase relations in the ternary system Ca-Ir-O were investigated at 1300 K. The only stable oxide detected along the binary Ir-O was IrO2. Three ternary oxides, CaIrO3, Ca2Ir4, and Ca4IrO6, compositions of which fall on the join CaO-IrO2, were found to be stable. Each of the oxides coexisted with pure metal Ir. Therefore, five working electrodes were prepared, consisting of mixtures of Ir + IrO2, Ir + IrO2 + CaIrO3, Ir + CaIrO3 + Ca(2)IrO4, Ir + Ca2IrO4 + Ca4IrO6, and Ir + Ca4IrO6 + CaO. These mixtures unambiguously define unique oxygen chemical potentials under isothermal and isobaric conditions. A novel apparatus, incorporating a buffer electrode between reference and working electrodes to absorb the electrochemical flux of oxygen through the solid electrolyte, was used for measurement. The buffer electrode prevented polarization of the measuring electrode and ensured accurate data. The standard Gibbs energies of formation of the compounds, obtained from the emf of the cells, can be represented by the equations IrO2 Delta(f)G degrees/J mol(-1) = -239, 230 + 172.19 T (+/- 240) CaIrO3 Delta(f(ox))G degrees/J mol(-1) = -29, 460 + 1.13 T (+/-380) Ca2IrO4 Delta(f(ox))G degrees/J mol(-1) = -35, 145 + 0.81 T (+/- 410) Ca4IrO6 Delta(f(ox))G degrees/J mol(-1) = -37, 400 - 2.01 T (+/- 630) where Delta(f(ox))G degrees represents the standard Gibbs energy of formation of the ternary compound from its component binary oxides CaO and IrO2. Based on the thermodynamic information, chemical potential diagrams for the system Ca-Ir-O were developed.