Adsorption and adsorbed species of SO2 in humid air during its oxidative removal were studied over pitch-based activated carbon fibers (ACFs) to clarify the reaction scheme of continuous removal of SO2, influence of reaction variables, and origin of activity enhancement by the calcination in inert atmosphere. The temperature-programmed desorption of SO2 adsorbed species suggests that when both oxygen and water were present, the main species on ACF was hydrated SO3 (H2SO4), which desorbed in the form of SO2 around 300 degrees C, and that the presence of both increased the amount of species by oxidizing and hydrating the adsorbed SO2, which desorbed around 60 degrees C without oxidation and 200 degrees C with oxidation but without hydration. The amount of adsorbed H2SO4 over ACF stayed at the same level after 25 h of reaction by balancing elution and adsorption of H2SO4 to allow the steady removal of SO2, More water enhanced the adsorption of SO2 at the initial stage but reduced the amount of adsorbed H2SO4 at the steady state, eluting out H2SO4 faster than the adsorption of SO2 to keep more active sites open over ACF at the steady state. Hence, complete removal was achieved. A higher calcination temperature significantly increased the rate and amount of both oxidative and oxidative/hydrating adsorption, explaining the largest activity of ACF calcined at 1100 degrees C. The amounts of adsorbed SOz and evolved CO from OG-20As by calcination at 500-1000 degrees C are well correlated, indicating that the evolved CO creates an adsorption site on the ACF surface, A similar but different correlation was obtained among ACFs of different surface areas calcined at different temperatures.