We have used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to investigate the adsorption of CO on PtRu/Ru(0001) near surface alloys (NSA) at 300 K and pressures from ultrahigh vacuum to 0.04 Torr. We observe differences in the fraction of Pt covered by adsorbed CO with changing Pt concentrations (0.36, 0.73, and 0.94 ML) in the NSA. For all alloy compositions and CO pressures, the amount of CO adsorbed on Pt sites in the alloy is less than what is observed on pure Pt(111). Further, the fraction of Pt sites covered by CO on the 0.36 and 0.73 ML Pt NSA surfaces are similar but lower than on the 0.94 ML Pt NSA surface. These observations support the concept of a decrease in the local adsorption energy of CO on Pt sites in alloy surfaces compared to pure Pt(111). We also found a correlation between the fraction of Pt covered by CO with the binding energy of the Pt 4f7/2 core level: the fraction of Pt covered by CO decreases with increasing Pt 4f7/2 binding energy. This observation may provide a s...