NO adsorption and reduction with NH3 over activated coke at low temperatures were studied by temperature-programmed desoption (TPD) and step-response experiments. NO adsorption is inhibited by competitive adsorption of NH3 in the absence of oxygen, and is significantly increased in the presence of oxygen. This phenomenon cannot be explained by either the oxidation of NO in the gas phase, nor the creation of oxidized surface on the activated coke. A hypothesis is proposed to interpret the phenomenon, which suggests that on the surface of the activated coke there are at least two types of adsorption sites, one of them adsorbs NO2 and/or the oxidized NO species rather than NO. NO-NH3-O-2 reaction proceeds via both the ED mechanism and the LH mechanism. NO conversion decreases with increasing temperature in the range of 30-250 degrees C, which suggests that the adsorption of reactants, especially NH3, is the rate-limiting step.