Iridium (Ir) could potentially improve ethanol electro-oxidation to CO2, employing CO species (COad) to be the key C1 intermediates. Thus, it is essential to carry out the investigation on CO adsorption and oxidation on the Ir surface. With regard to CO adsorption, in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) shows that only linearly adsorbed COad species (COL) are detected at ca. 2010–2060 cm–1. With a continuous dosing of CO, the open-circuit potential (OCP) of the Ir electrode decreases to ca. 600 mV, being associated with the displacement and/or consumption of surface OHad species. For CO oxidation, the onset oxidation potential is at ca. 0.64 and 0.55 V versus reversible hydrogen electrode in acidic and alkaline media, respectively. Meanwhile, the Stark tuning rates of ca. 29 ± 1 cm–1 V–1 (acidic media) and 34 ± 1 cm–1 V–1 (alkaline media) suggest an approximate COL monolayer (especially in acidic media), and a stable COL–H2Ofree co-structure could be formed. Therefore, CO oxidation might start from the edges and/or defect sites of the COL–H2Ofree co-structure, following the “nucleation and growth” kinetic model of the Langmuir–Hinshelwood mechanism. This work may provide new insights into understanding CO adsorption and oxidation on the Ir electrode surface.