Glassy carbon (GC) electrodes were activated by electrochemical constant potential anodization in order to generate high-surface area, high-capacitance electrodes. After anodic oxidation in sulfuric acid the electrodes exhibited increased capacitance. After subsequent electrochemical reduction of the activated layer, a further significant increase in capacitance was observed. Growth, structure, and surface properties of the activated electrodes were monitored by cyclic voltammetry, differential electrochemical mass spectrometry, spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Two different types of glassy carbon obtained by pyrolysis at 1000 degrees C and at 2200 degrees C were compared. Differential electrochemical mass spectrometry reveals that CO2 is the main reaction product during oxidation, while CO2 and Hz are detected during reduction. The values of surface layer capacitance and thickness determined by spectroscopic ellipsometry increase as linear functions of oxidation time, rue resulting volumetric capacitance was at least 100 F/cm(3). After oxidation, the presence of functional surface groups was demonstrated by XPS. The relative contributions of the different surface functionalities depend on the pyrolysis temperature of the GC. Reduction lowered the concentration of oxygen-containing functional surface groups. The XPS results were qualitatively confirmed by Fourier transform infrared measurements carried out at the same samples. AFM measurements on glassy carbon showed that the film growth both into and out of the substrate, resulted in a raised surface after activation. A qualitative model for film growth is presented.