This work reported the adsorption and reaction performance of FeOx supported subnanometer cluster and single-atom Ir catalysts for the oxidation of CO at low temperature. By varying the pretreatment temperature and Ir loading, the single-atom and subnanometer cluster Ir catalysts were obtained. The Ir subnanometer clusters exhibited higher activity for the oxidation of CO with or without the presence of H-2 than the single-atom counterpart. By using adsorption microcalorimetry and in situ infrared spectroscopy measurements, it was found that the Ir subnanometer clusters not only promoted the adsorption and reaction of CO and O-2 but also facilitated the formation of OH species from reaction between H-2 and O-2, thus opening a new reaction pathway between CO and OH species to produce CO2 compared with that between CO and O species on the single-atom counterpart. (C) 2017 American Institute of Chemical Engineers