Alumina, silica and beta zeolite supported iridium catalysts were prepared by atomic layer deposition (ALD) from two different metal precursors, Ir(acac)(3) and Ir(thd)(COD). The use of Ir(thd)(COD) in ALD is reported for the first time. The aim was to investigate the effect of the precursor on catalyst surface species, chemical state and characteristics. Controllable ALD reaction was successful with both iridium precursors on alumina and with Ir(acac)(3) on beta zeolite. On these catalysts, iridium particle sizes were very small (1-3 nm). Instead, some thermal decomposition of both precursors was observed during deposition on silica. At conditions, where no or very little decomposition of the precursors took place, the differences in the chemical state and characteristics of the as-prepared Ir/support samples were negligible, In ALD, Ir(acac)(3) is slightly more stable at high deposition temperatures (> 200 degrees C) while Ir(thd)(COD) enables the utilization of larger temperature range since it vaporizes at lower temperature compared to Ir(acac)(3). The results thus indicate that Ir(thd)(COD) is a suitable new precursor for ALD. (C) 2010 Elsevier B.V. All rights reserved.