The adsorption and electrooxidation of CO on polycrystalline Ag at pH 13, 9.2, 7, and 0.3 have been studied by cyclic voltammetry and Fourier transform infrared spectroscopy (FTIRS). At the four pH values the cyclic voltammograms (CVs) showed the formation and electroreduction of underpotential (UP) Ag oxides, i.e., oxides formed at potentials lower than the equilibrium potential of the Ag/Ag2O couple. Two pairs of peaks of UP oxides appeared at pH 13 and 9.2, but only one pair at pH 7 and 0.3. At pH 13 and 9.2 the current density of the anodic peak of the UP oxide appearing at the more positive potential increased if the solution was saturated with CO, and its peak potential increased (pH 13) or remained the same (pH 9.2), indicating the electrooxidation of adsorbed CO. Both at pH 7 and 0.3 the anodic peak in the presence of CO appeared at potentials 0.2 V more negative than that in the corresponding base electrolyte. Only at pH 13 did the CO adsorbed on Ag remain adsorbed after eliminating the dissolved CO by N-2 bubbling, indicating that CO was strongly adsorbed. The FTIR spectra showed, at the four ph values, a band at 1970-2000 cm(-1) assigned to CO linearly adsorbed on Ag and, at neutral and alkaline pH, a band at 1860-1900 cm(-1) assigned to CO adsorbed in. bridge position. At pH 9.2, 7 and 0.3 there appeared a band at 2048 (pH 9.2 and 7) or 2112 (pH 0.3) cm(-1) whose frequency did not change with potential and whose intensity increased with increasing potential and which was therefore assigned to CO adsorbed on an UP Ag oxide.