Carbon monoxide adsorption and interactions with electrolyte species were examined for a Pt(111) electrode in 0.1M HClO4. The experiments were conducted with an ex situ ultrahigh vacuum (UHV)-electrochemical system, with CO being adsorbed either from the vapor phase in the vacuum chamber or from solution. CO oxidation coulometry and cyclic voltammetry were used to characterize CO coverage in solution, and thermal desorption spectroscopy was used to measure CO coverage in vacuum, desorption kinetics and to detect coadsorbed electrolyte species. In agreement with earlier studies, the saturation coverage of 0.68 ML of CO from solution dosing is nearly 40% greater than the saturation coverage of 0.50 ML in vacuum al room temperature. The higher saturation coverages survive transfer to vacuum, but only in the presence of coadsorbed electrolyte species (H2O and ClO4) retained after removal of the electrode from the electrolyte. In the absence of coadsorbed electrolyte species, saturated, solution dosed CO transferred to vacuum exhibits the same coverage as vapor dosed CO. Interaction between CO and electrolyte species was confirmed through detection of both in thermal desorption following immersion of a vapor dosed CO adlayer into solution and back-transfer to vacuum. Kinetic modeling of CO thermal desorption showed that, regardless of whether CO is adsorbed from solution or from vapor, the CO-CO repulsive interactions are approximately 40% less when electrolyte species are retained than when they are absent. (C) 1997 Elsevier Science B.V.