The effect of surface functionalities, specific surface area and pore size of activated carbon fibers (ACFs) on the adsorption of toluene and acetaldehyde, two volatile organic compounds (VOC), at low concentrations (similar to 80 ppmv) and short contact time (20 ms) has been studied. Two different types of ACFs characterized by low temperature N-2 adsorption: ultramicroporous (d (pore) < 1 nm) and supermicroporous (d (pore) similar to 1-2 nm) were tested. Both ACFs were effective for the removal of toluene attaining the adsorption capacity as large as 51 wt%. The surface chemistry of ACFs (O-containing functional groups) was characterized by temperature-programmed desorption monitoring the CO/CO2 evolved. Oxidative treatment of ACFs by nitric acid increased the surface concentration of O-groups. This resulted in lower adsorption capacity towards toluene but higher one towards acetaldehyde. This result was rationalized based on different type of VOC interactions with the carbon surface.