The electroreflectance (ER) technique has been applied to the study of a Pt(111) electrode in different acidic and alkaline electrolytes. The variation in the normalised potential derivative of the reflectance signal, 1/R(dR/dE) is observed in sulphuric and perchloric acid media as the potential is scanned 1.0 V positive from the hydrogen evolution region. In both these media, the usual hydrogen adsorption-desorption region corresponds to a zone in which the ER signal is practically zero and does not change with the potential. However, in the anomalous region, an increase in the ER signal was observed indicating that here, the adsorbed species increased the electron density of the platinum surface. The ER spectrum recorded for the cyanide-covered Pt(111) surface showed a large decrease in the electron density of the platinum surface and this is attributed to electron back-donation from the platinum to the adsorbed cyanide. In alkaline solutions, the integrated ER spectrum showed relatively small changes which corresponded to the features in the cyclic voltammogram and were attributed to the reorientation of the hydrogen-bonded surface species and water dipoles.