The effect of the micropore structure of activated carbon fibers (ACFs) on the adsorption of toluene at low concentration (10-80 ppmv) was studied over two types of ACFs. Both adsorbents presented similar surface chemistry but different porous structure: one ACF was ultramicroporous (d (pore) < 1 nm) and the other supermicroporous (d (pore) similar to 1 to 2 nm). Toluene adsorption isotherms were determined for both ACFs and were found to be consistent with Dubinin-Radushkevich (D-R) model. The toluene adsorption enthalpies calculated from temperature-programmed desorption profiles at temperatures below 330 K were close to the values obtained from D-R equations. Larger adsorption strength was found in the ultramicroporous adsorbent as compared to the supermicroporous one suggesting that the pore shape strongly influences the adsorption mechanism. The adsorbent with a lower specific surface area but narrower micropores could be more efficient at high temperatures than an adsorbent with large pore volume and wider micropores. The findings reported have a big importance for correct choice of material when designing efficient structured adsorbing bed.