Using different isotopologues of the reactant gases CO and O-2, infrared reflection absorption spectroscopy (IRAS) has been used to investigate the transient surface species on the Pt{100} surface under reaction conditions which was first shown to give rise to an absorption band around 1630 cm(-1) by Hong and Richardson (J. Phys. Chem. 1993, 97, 1258). The results show that this band cannot be attributed to a C-O stretching frequency of the CO from the gas-phase incorporated into a CO-O surface complex, such as that identified as the transition state in recent density-functional theory (DFT) calculations of the Pt{111}/CO + O-2 and Pt{100}/CO + NO reactions. The IRAS results are consistent, however, with a surface O-C-O species of low symmetry in which the IR band is due to a C-O stretching mode involving an O atom arising from the molecular O-2, and estimates of the desorption energy of this species show it is chemisorbed. This surface intermediate may also be involved in the CO + NO oxidation reaction over Pt{100}, but the steady-state coverage at the higher reaction temperature would preclude its observation in IRAS. The results suggest that further DFT calculations exploring alternative reaction paths may be of value.