The adsorption behaviour of CO on a Pt electrode was investigated in 1 M HClO4 solution by the use of the in situ specular reflectance method, Simultaneous measurement of the reflectivity of the Pt electrode and the suppressed current of hydrogen adsorption enabled us to find the relation between the reflectivity change and the coverage of adsorbed CO. The reflectivity was found to be dependent on the amount of coverage. In the low coverage region, it increased with the increase of the coverage, while in the high coverage region, it decreased rapidly. The reflectivity profile dependence on CO coverage suggests the presence of more than two different types of adsorbed CO. The present results are in accordance with the results obtained with IR measurements, which suggest the presence of reflection absorption spectroscopic inactive CO.