In surface science, CO is a suitable probe to test the properties of metals in different chemical environments. We have analyzed CO adsorption on different PdAu alloys at both low and medium Pd content through a theoretical approach. CO is adsorbed on several sites, and the ensemble rules both the CO stretching frequency and the binding energies. Multiple CO adsorption on large ensembles (dimers) is likely at very low temperatures or high CO pressures. As a consequence, at low temperatures, the vibrational spectra of CO on the PdAu(111) alloy can lead to the identification of dimers as singletons. This can lead to misinterpretations of the nature of the sites and the wrong identification of ensembles that are fundamental both in chemical and electrochemical processes. According to our first principles based thermodynamic model, CO-induced segregation is possible for PdAu(111) at moderate CO pressures, about 10^-2Torr.