SO2 removal activity at 100-degrees-C of three kinds of poly(acrylonitrile)-based active carbon fibers (PAN-ACFs) and their heat-treated analogues, FE-100-600, FE-200-800, and FE-300-800 (the heat treatment temperatures were 600 and 800-degrees-C), were studied, using a model flue gas containing 1000 PPM SO2. SO2 was removed rapidly and completely for 4.2,7.0, and 5.1 h after SO2 adsorption started, using FE-100-600, FE-200-800, and FE-300-800, respectively. When FE-200-800 and FE-300-800 were regenerated in the temperature range of 400-600-degrees-C, SO2 adsorption capacities decreased very drastically. When the regeneration temperature was raised to 800-degrees-C or higher, the SO2 adsorption capacity decreased more slowly, but large weight losses of the ACFs were observed. On the other hand, when FE-100-600 was regenerated at 600-degrees-C, the SO2 adsorption capacity was very stable with a smaller loss of carbon than observed with FE-200-800 or FE-300-800. Oxygen functionalities which disturb SO2 adsorption are suggested to be generated during regeneration, because adsorbed H2SO4 was decomposed to SO2 reductively, with oxygen remaining on the surface of ACF. These oxygen functionalities on FE-100-600 were decomposed almost completely to CO2 below 600-degrees-C. In the case of FE-200-800, 60% of the surface oxides were decomposed to CO2 at 200-400-degrees-C, and 40% were decomposed to CO at 400-900-degrees-C. This is the reason why FE-100-600 was regenerated effectively below 600-degrees-C with a small loss of carbon. The adsorption capacities of ACFs for H2SO4 (SO2 in the presence of O2 and H2O) were correlated to those for SO2 in the presence of O2 and for H2O on the same ACFs to discuss the roles of oxygen groups on the surface. The adsorption of SO2 alone was not corrected, although 60% of the SO2 was estimated to be oxidized on an ACF surface with remaining oxygen.