The decomposition of (5-methylcyclopentadienyl)(1,5-cyclooctadiene) iridium, (MeCp)Ir(COD), has been studied on an O-covered Rh surface by isothermal and temperature-programmed desorption and isothermal reaction spectroscopy using time-of-flight mass spectrometry, and by Auger electron spectroscopy. After dosing at 100 K, monolayer molecularly adsorbed (MeCp)Ir(COD) desorbs at 325 K competing with decomposition to form adsorbed McCp and GOD. We infer that these react with adsorbed oxygen to form O-containing organic surface-bound fragments that decompose to desorb only CO2, CO and H2O since no other O-containing products were detected in the 3D spectra. While dosing both (McCp)Ir(COD) and O-2, pure Ir film growth occurs above 500 K provided there is a large excess of O-2. When dosing these two reactants at 450 K, O, C, H and Ir all accumulate to give a slowly growing film that slowly evolves CO but negligible H-2, H2O or CO2.