Reduction of very low concentration NO with NH3 at room temperature was examined over active carbon fibers (ACF) derived from pitch (P) and poly(acrylonitrile) (PAN), in order to find a way for cleaning the humid atmosphere of 10 ppm NO. As-received (not calcined) ACFs exhibited considerable activity in dry air by converting 50-65% of NO by W/F (weight of fiber/gas flow) = 5 x 10(-3) g . min . mL(-1) however, humidity especially over 50%, inhibited the reduction very severely. The conversions were 5% over both P-ACF and PAN-ACF at 80% relative humidity (rh). The calcination of P-ACF under N-2 flow at 800 degrees C increased the activity slightly in dry air, and significantly in the wet air. The conversions over P-ACF calcined at 800 degrees C were 25% in 80% rh and <5% in 100% rh. The activity of PAN-ACF in dry air decreased markedly with higher calcination temperature, while the highest activity in the wet air was obtained by the calcination at 600 degrees C, both lower and higher temperatures reducing the activity, although the conversion was only 12% in 80% rh. Ability of ACFs for NO and NH3 adsorption did not always correspond to their activity, indicating that a very small number of active sites work against the substrates of low concentration. The activity in wet air appears to reflect the hydrophobic natures of the ACF surfaces since water vapor inhibits the reaction and the adsorption of NO. The calcination removes the major oxygen functional groups from the ACF surface, emphasizing the hydrophobic nature of the surface. The defects left by the removal of oxygen functional groups may enhance the catalytic activity of remaining active sites.