The chemisorption of CO on a Rh electrode in 0.5 M HClO 4 and 1 M NaOH was studied by cyclic voltammetry and UV-Visible Potential-Modulated Reflectance Spectroscopy (PMRS). Electrooxidation of dissolved CO on Rh is negligible in acid. On the contrary, the stationary current in base reaches ∼- 1 5 of the diffusion-limited value. We report a voltammetric peak due to the oxidation of dissolved CO in base, which can be observed only for CO admission potentials < 0.1 V vs. RHE, and which precedes the sharp peak for oxidation of (chemisorbed and dissolved) CO. The potential of this sharp peak in base is independent of the admission potential up to E adm = 0.47 V, but for higher values the peak potential increases linearly with E adm, probably due to the formation of a rhodium oxide prior to CO admission. The amount of CO chemisorbed on Rh in base is independent of the presence on the electrode surface of a thick layer of electrochromic oxide previously grown by potential cycling, which we attribute to CO chemisorption continuing to take place on the unoxidized Rh surface. The PMR spectrum of Rh on which CO is chemisorbed shows a well-defined maximum, which appears at 265 and 295 nm in acid and base, respectively. This PMRS maximum is due to an electric field effect, since its height is independent of the modulation frequency, and grows linearly with the modulation amplitude. Reflectograms recorded at the wavelength of the PMRS maximum during a slow potential sweep prove conclusively that it originates in the chemisorbed CO. The PMRS maximum at 265 nm is attributed to linearly chemisorbed CO, and that at 295 nm to bridged CO.