The interaction of CO and Xe on Pt(335){Pt[4(111)x(100)]} has been studied by temperature-programmed desorption (TPD) and infrared reflection-absorption spectroscopy (IRAS). TPD results show desorption peaks at similar to 400 K and similar to 500 K from the pure CO layer. The high-temperature peak is related to CO adsorbed on step sites, while the low-temperature peak is related mainly to CO adsorbed on terrace sites. IRAS studies, performed on CO layers after annealing to 290 K, show four subsequently appearing vibrational modest similar to 2075 and similar to 1880 cm(-1) for low theta(CO), and similar to 2092 and similar to 1900 cm(-1) for high theta(CO) Experiments involving Xe coadsorption and isotopic CO site exchange have been conducted, and the four vibrational modes are assigned to terminal and bridged CO adsorbed on step sites for low theta(CO) and on terrace sites for high theta(CO), respectively. The stepwise sequence for CO adsorption on Pt(335) is thus : terminal-bound CO followed by bridge-bound CO on step sites at low theta(CO), then terminal-bound CO followed by bridge-bound CO on terrace sites at high theta(CO). Altogether, this study demonstrates the usefulness of Xe coadsorption in identifying adsorption sites as well as the adsorption sequence.