The adsorption of CO on three different gold nanoparticle catalysts supported on high surface area TiO2 was studied using infrared transmission spectroscopy at room temperature and CO pressures typically used in CO oxidation reactions. The three, real-world catalysts were Au catalysts synthesized in our laboratory from thiol monolayer protected clusters (MPCs) and two commercial catalysts from the World Gold Council (WGC and AuTEK). Within experimental reproducibility, the adsorption data for the three catalysts are indistinguishable. While showing approximately Langmuir behavior, the adsorption data also show coverage dependence, as others have observed for many catalyst systems. Two approaches were used to fit the data, a two-site model and a variable binding constant model. The two-site Langmuir model yielded strong (36%) and weak (64%) binding constants of 2740 and 146 atm-1, respectively. Alternatively, using a sliding-tangent Langmuir fit gave a variable binding constant of 2670-120 atm-1 at room temperature for coverage θ = 0–0.8. The heat of adsorption was then extracted from the binding constants using a literature value for –TΔS. These values were determined as ΔH = –64 and –56 kJ/mol for strong and weak binding according to the two-site model and ΔH = –63 to –56 kJ/mol for coverage θ = 0–0.8 for the variable binding constant model. These values agree well with literature values obtained (i) using supported catalysts under higher pressures and (ii) using model catalysts under higher pressures and ultrahigh vacuum conditions.