Abstract Formation of IrO 2 and Ir by thermal decomposition of iridium(III)-acetylacetonate and IrO 2 from iridium(IV)-oxide dihydrate has been investigated by XRD, thermal analysis, FT-IR and laser Raman spectroscopy. Iridium(III)-acetylacetonate decomposed in air at 200 °C, yielding Ir and traces of IrO 2. An increase in heating temperature led to an increase in IrO 2 fraction. A gradual increase in crystallite size of Ir from 11(3) to 30(7) nm with the increase of temperature from 200 to 550 °C was measured, whereas the crystallite size of IrO 2 changed from 12(4) to 20(5) nm in the range between 350 and 550 °C. Ir film formed at 550 °C contained crystallites of several tens of nanometers. Iridium(IV)-oxide dihydrate showed an amorphous XRD pattern. Upon heating in air at 600 °C for 2–24 h, crystalline IrO 2 as a single phase was produced. Broad IR band at ∼550 cm −1 was recorded due to ν(Ir–O) vibrations. Raman spectrum of IrO 2·2H 2O showed two broad bands at 708 and 744 cm −1, which could be assigned to IrO 2. An additional broad band at 352 cm −1 was also visible. The nature of Raman band at 352 cm −1 was interpreted in terms of poor crystallinity and/or the presence of very fine IrO 2 particles that may tend to periodic aggregation. IrO 2 produced at 600 °C showed Raman bands at 718 and 544 cm −1. These wave numbers are smaller than those observed for IrO 2 single crystal.