The vibrational features of carbon monoxide (CO) adsorbed on Ir(111) were studied by means of high-resolution core-level spectroscopy. By monitoring the Ir 4f 7/2 core level as a function of exposure, we proved that the CO adsorbs on the surface always in on-top sites, in agreement with the results of vibrational spectroscopy techniques and density functional theory studies. The C 1s vibrational splittings measured for the p(√3 × √3)R30°(233.4 ± 0.5 meV) and c(4 × 2√3)rect (231.4 ± 0.4 meV) structures were in good agreement with the Z + 1 model. Despite the very small error bar of the measurements, it was not possibile to reveal any anharmonic contribution to the spectral lineshape. We speculate that the contribution of the unresolved vibrational mode of the frustrated translation or the effect of phonon-mediated interaction with the substrate can account for the observation of this outcome.