A study has been made by x-ray diffraction analysis of the reactions that occur in an air environment between Ir and IrO2 and each of the following: Nd2O3, Sm2O3, Eu2O3, Gd2O3, Dy2O3, Ho2O3, Y2O3, Er2O3, Tm2O3, Yb2O3, Lu2O3, In2O3, Sc2O3, and A12O3. In air Ir oxidizes at low temperatures to form IrC2 which in turn dissociates at 1020 °C. The pseudo binary Nd2O3-IrO2 was studied in detail inasmuch as it typified many of the Ln2O3., -IrO2 systems. Two compounds, Nd2O3.-2IrO2 and 3Nd2O.-2IrO2 occur in the system. The former, a cubic pyrochlore type phase, dissociates upon heating at 1190 °C. The 3:2 compound dissociates to the solid phases, Nd203., and Ir, at 1300 °C. Prior to dissociation, the 3:2 compound undergoes an apparent polymorphic transition at 1195 °C which may be related to an oxygen loss. Up to at least 2000 °C no further reaction occurred between Ir and Nd2O3,. All B-and C-type rare earth oxides formed cubic pyochlore type compounds with IrO2. Each of these compounds subsequently dissociated upon heating. No apparent reaction occurred between lrO2 and either In2, O3, Sc2O3, or Al2O3.