The adsorption and electrooxidation of CO on Co at four pH values : 14, 9.2, 6.8, and 3 have been studied by cyclic voltammetry and Fourier transform infrared spectroscopy. Over this wide pH range, 3-14, the main species chemisorbed on Co was linear CO. At pH 14 and 9.2, and possibly also at pH 6.9, a small amount of multibonded CO was also present. From the intensity of the infrared bands it was estimated that a sizable coverage of the Co surface with CO was reached, which is in agreement with the inhibition of cathodic Hz evolution and anodic Co electrooxidation observed in the presence of CO over the whole pH range 3-14. The frequency of CO stretching of linear CO chemisorbed on Co was, with good approximation, dependent only on the potential versus a pH-independent reference electrode, and not on the pH of the solution. This independence on the pH value was attributed to the exclusion of water and its ions from the inner part of the double layer by chemisorbed CO.