The catalytic oxidation of sulfur dioxide in flue gases over activated carbon catalysts was studied. Catalysts made from different materials or by different methods were markedly different. The catalysts were characterized by BET isotherms and X-ray power diffraction. The acidic and basic sites and/or strength on the catalysts were determined by temperature-programmed desorption, while surface functional groups were determined by X-ray photoelectron spectroscopy. The catalytic activity depended on oxygen-containing surface functional groups with Bronsted basicity properties. These groups on a furfural residue activated carbon could be increased by a suitable treatment to enhance the catalytic activity but other catalysts in this study showed no activity enhancement with a similar treatment.