The catalytic activities of moderately activated pitch-based carbon fibers (ACFs) were examined for the reduction of low concentration NO (10 ppm) with ammonia in dry and wet (80% humidity) air. A particular ACF calcined at 850 degrees C exhibited very high activity even in wet air, providing stationary NO conversions of 76 and 46% in dry and wet air, respectively, at the contact time W/F = 5 x 10(-3) g min mL(-1). An optimum NH3 concentration of 15 ppm gave the largest NO conversion and the lowest leakage. The adsorptive abilities of the ACFs for NO and NH3 do not directly correlate with their catalytic activity, indicating that all adsorbed species are not active for the reduction. Catalytic activity was enhanced by calcination in an inert atmosphere, while reduced adsorptive ability suggests that the unsaturated carbon valencies induced by the elimination of oxygen functional groups act as additional active sites over the ACF surface. The hydrophobic nature, which is also enhanced by the calcination, is essential in moderating the retardation by H2O of both NO adsorption and activation. The moderately activated ACFs of 700-800 m(2)/g surface area after the heat treatment at 800-850 degrees C appear to contain the active sites of highest activity situated on the hydrophobic surface within pores not occupied by condensed H2O.