Abstract: Oxygen functional groups on a commercial activated carbon were modified by (NH₄)₂S₂O₈ oxidation and subsequent annealing at different temperatures. The effects of the surface modification on the electrochemical performance as supercapacitor electrodes were investigated by XPS, FTIR, argon adsorption and electrochemical tests. Results indicate that the oxygen functional groups, especially carboxyl and carbonyl groups improved the wettability of the pore surfaces, increased the electrolyte diffusion rate into the electrode and increased the specific capacitance by an additional pseudo-capacitance in a 6 mol/L KOH aqueous electrolyte. An excess oxygen content blocked the pores, leading to poor electrochemical performance, but annealing at 300 ℃ in an inert atmosphere increased the specific capacitance and improved the rate performance in a 6 mol/L KOH aqueous electrolyte. In a 1 mol/L Et₄NBF₄/PC organic electrolyte, however, oxygen functional groups introduced by (NH₄)₂S₂O₈ oxidation reduced the specific capacitance and annealing at 700 ℃ to reduce their content effectively increased the specific capacitance.