Further insight into important aspects of the chemical surface properties of activated carbons is provided by rationalizing the differences in O-2 adsorption and desorption behavior of hydrogen- and nitrogen-treated activated carbons. The effects of heat treatment on the electrochemical behavior of these carbons were also instrumental in elucidating the nature and the distribution of carbon active sites, Activated carbon surfaces stabilized with hydrogen at 950 degrees C adsorb very little O-2 L at room temperature but the graphene layers are terminated with many free carbon active sites because significant O-2 adsorption does take place at 150 degrees C. Furthermore, the low-coverage differential heats of adsorption on sites accessible at 150 degrees C were lower than those on sites accessible at 25 degrees C. The role of these free carbon sites in determining the basicity of activated carbons was also addressed. On the basis of the important finding that the point of zero charge exhibits a maximum at intermediate heat-treatment temperatures, it is proposed that-in addition to the delocalized basal-plane pi electrons-the localized pi electrons at graphene edges (e.g., inplane divalent sigma pairs) act as Lewis bases that interact with protons in aqueous solution.