Effects of temperature and CO partial pressure on the adsorption of carbon monoxide at a polycrystalline platinum electrode in 0.1 M HClO4 were studied with cyclic voltammetry and infrared spectroscopy. Appreciable CO coverages were detected with infrared spectroscopy from ambient up to the high-temperature limit of the experiments (75°C). In CO saturated solution, voltammograms were dominated by the diffusion-controlled current arising from the oxidation of bulk CO. However, under these conditions the adsorbed CO layer was readily detected with infrared spectroscopy. At 75°C, a strong atop CO band (2075-2080 cm-1) was observed at double layer and hydrogen adsorption potentials. Loss of the adlayer due to oxidation occurred at potentials positive of 0.3 V (vs KCl saturated Ag/AgCl). In CO-free electrolyte solution, atop CO band intensities measured at 75°C were lower than those recorded in CO saturated solution by about 50%, and the adlayer was more easily oxidized. CO formed from solutions of 0.3 M methanol in 0.1 M HClO4 was detected at 25 and 50°C. Temperature did not have a strong effect on the equilibrium methanolic CO coverages under these conditions.