The oscillatory reduction of hydrogen peroxide on an n-type gallium arsenide electrode was studied by means of the electrolyte electroreflectance technique. It is found that during the current oscillations both the reflectivity and electroreflectance oscillate with qualitatively different patterns. A simple model, which attributes the current oscillations to an anomalous dependence of the band bending in the semiconductor depletion layer on the potential drop across the semiconductor/electrolyte interface, is considered for comparison. It is concluded that the absolute reflectivity detects a slow variable of the system, most likely related to the chemical composition of the surface, and the electroreflectance detects a fast variable of the system, most likely related to the interfacial potential distribution.