The mechanisms of the SO2 adsorption properties of activated carbon fibers (ACFs) after plasma treatment were studied. The surfaces of the ACFs were modified by a plasma treatment using N-2 gas to enhance the SO2 adsorption of the fibers based on the effects of introduced nitrogen functional groups, especially quaternary nitrogen groups. Through the comparative analysis of X-ray photoelectron spectroscopy (XPS), especially the N1s component and SO2 adsorption data, quaternary nitrogen groups were determined to be effectively introduced onto the surface of the ACFs. SO2 molecules have lone pair electrons and attach themselves to the ACFs' pores as the concentration of quaternary nitrogen increases. However, the pyridine N-oxide (N-X) functional group exhibited acidic properties and had a neutralizing effect on the ACF surfaces, reducing the strength of electrostatic interactions with the SO2 molecules. These types of mechanisms are proved by various physicochemical and electrical characterizations, especially the SO2 sensing capability of plasma-treated ACFs. The reversible sensing indicates that N-2 plasma treatment changed the electrical properties of the carbon sample, leading to an intriguing sensing mechanism. (C) 2016 Elsevier B.V. All rights reserved.