Polished and electrochemically pretreated glassy carbon electrodes have been investigated using impedance spectroscopy in 1 M KCl electrolyte and scanning tunneling microscopy in ambient air. For the impedance data, an equivalent circuit with contributions of distributed capacitancies of both basal and edge planes of glassy carbon was proposed. Moreover, a pure capacitance deriving from partially dissociated carboxyl groups at the edge planes was postulated. Electrochemical oxidation leads to an increased distributed capacitance, which may be attributed to the edge planes as well as to the pure capacitance attributed to the carboxyl groups. Impedance measurements suggest that both roughening and smoothening occur during treatment processes. This is confirmed by scanning tunneling microscopy images which show simultaneous corrosion of single structures at the surface and smoothening of the overall surface.