The CO coverage of a Pt{110} surface in both the high and low reaction rate branches of the bistable CO oxidation reaction has been determined by Infrared Reflection-Absorption Spectroscopy (IRAS), first performing extensive calibration experiments on the various factors determining the absorbance and frequency associated with the C-O vibrational stretching mode. The same two states of the surface are shown to be present under steady-state low and high reaction rates and when the surface is undergoing pattern formation and homogeneous reaction rate oscillations. Using the CO coverages determined by IRAS, the intensities observed in a series of photoelectron emission microscopy images have been used to elucidate the oxygen coverage in both coadsorption states. The low reaction rate branch is found to be associated with a high CO coverage (0.5+/-0.1 ML) and very low O coverage (0.03+/-0.01 ML) consistent with the (1x1) unreconstructed phase. In the high rate branch the surface has a low CO coverage (0.05+/-0.03 ML) and O coverages in the range 0.3-0.7 ML [(1x2) reconstructed phase]. No evidence for bridged CO, oxide, or subsurface oxygen, variously proposed to play a role in the reaction rate bistability, was found under the conditions measured. These findings are consistent with the site blocking and reconstruction model. Coadsorption experiments of CO and oxygen under nonreactive conditions, performed as part of the IRAS calibration process, demonstrate that CO and O can occupy a mixed adlayer and identify two different chemical environments for CO adsorption.