The reactivity of adsorbed NO (including NO2) and NH3 in the presence of 4.0% oxygen in He was examined over a pitch-based ACF calcined at 800 degrees C. Regeneration at 30 degrees C by 4% O-2 in He without NH3 was found to be optimum for the recovery of the initial activity with complete removal of NO within 3 h, with minimum leaks of adsorbed NO and NH3. A higher temperature of 40 degrees C for regeneration increased the liberation of adsorbed NO, and NH3 over ACF was rather slow at a lower temperature of 25 degrees C, slow regeneration being achieved. Oxygen appears necessary to regenerate the ACF through enhancing the reaction of adsorbed NO and NH3 for the initial activity, which was ascribed to the catalytic activity for NO-NH3 and adsorption of both NO and NH3. NH3 in the gas phase appears to inhibit the regeneration reaction of adsorbed species, by using the leaking amount during the regeneration.