Oxygen and substrate effects on iridium film growth have been investigated by metallorganic chemical vapor deposition (MOCVD), insitu X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Iridium tris-acetylacetonate, Ir(CH3COCHCOCH3)(3), was used as the Ir precursor. High purity CVD Ir films were obtained when this precursor was co-dosed with oxygen. Without oxygen, the Ir film contains noticeable carbon. The presence of oxygen not only removes carbon, but also prevents carbon incorporation from other reactive gas components, such as acetone. Oxygen also controls the film deposition rate and has a significant impact on the him morphology. Substrate effects on the initial growth rate an evidenced by the fact that the growth rate on a titanium-carbonitride (TiCN) surface is significantly higher than that on an SiO2 surface. Selective deposition in the presence of oxygen was also observed; compared to SiO2 and Ta2O5 substrates, no deposition (or very slow deposition) was observed on sapphire at 400 degrees C. The topography of films grown on the various substrates was compared using AFM. Ir was roughest when deposited on SiO2 using a low oxygen flow rate. This is because the initial growth of Ir films on SiO2 follows a three-dimensional growth mode on isolated islands.