We have studied the adsorption and electrooxidation of CO on Ni at four pH values : 14, 9.2, 6.9, and 3. Over this pH range bridge and Linear (on-top) CO coexist on the Ni surface. The ratio of bridge to linear CO as estimated from IR data, assuming that the ratio of the molar absorption coefficient of linear CO to that of bridge CO is 2.5, increased with pH, from 1:1 at pH 3 to 8:1 at pH 14. From the intensity of the bands of chemisorbed CO and/or CO2 produced by its electrooxidation a complete CO coverage of the Ni surface at the four pH values was estimated. In a plot of the stretching frequency of linear CO on Ni vs the electrode potential referred to a pH-independent reference electrode the data for the pH values 3, 6.9, and 9.2 clustered around a straight line, this pH independence of the stretching frequency of linear CO being attributed to an exclusion by chemisorbed CO of water and its ions from the surface of the metal. For bridge CO, although the data for pH 6.9, and 9.2 also clustered around a common Line in the CO frequency vs electrode potential plot, the data for pH 14 defined a straight Line shifted vertically toward lower frequencies by about 20 cm(-1). The Stark shift was 56 +/- 6 cm(-1) V-1 for linear CO at pH 3, 6.9, and 9.2 and 64, 56, and 79 cm(-1) V-1 for bridge CO at pH 6.9, 9.2, and 14. An important consequence of the pH independence of the CO stretching frequency at a given potential vs a pH-independent reference electrode is that the typical frequencies for linear and bridge CO are about 50 cm(-1) lower at pH 14 than at pH 0.