The coadsorption of hydrogen and carbon monoxide on the (111) face of a platinum single crystal has been studied by a combination of thermal flash desorption and electron impact desorption (EID) techniques in ultrahigh vacuum. Several compositions over the range from 0%–100% CO have been studied, over a total pressure range from5×10−9to5×10−7 Torr. The observed adsorption behavior is complex, indicating multiple binding sites for both gases. In addition, the EID studies show the formation of a surface species containing both H and CO. This same species has also been observed when formaldehyde was adsorbed directly from the vapor.