The oxidation of the surface of glassy carbon (GC) in 0.5 M KOH was studied by the method of electroreflectance (ER). For a freshly-prepared mechanically polished GC surface, the ER spectra have relatively low intensity and exhibit inversion of sign at the electrode potential -0.4 V. The ER spectra of the GC electrode change substantially after the electrochemical oxidation at E = + 0.6 V. Oxidation considerably increases the intensity of ER, and the band located at quantum energies from 1.8 to 2.4 eV (Band A) becomes much broader. The maximum of the band shifts towards higher energies. The ER spectra depend strongly on the electrode modulation frequency. It is concluded that anodic oxidation leads to the generation of surface states whose impact on the ER spectra of GC is especially important at potentials close to the pzc. Structurally, the surface states seem to be related to the generation of edge-type carbon atoms caused by oxidation. Broadening and the shift of Band A in the ER spectra of an oxidized surface are hypothesized to be a manifestation of optical transitions from the Fermi level to the surface states. The frequency dispersion shows that there are several surface states with distinct kinetic differences. These observations support the hypothesis that activation of the surface of carbon materials is accompanied by the generation of surface states.