The role of O- and N-containing surface species in the course of NO reduction over phenol-formaldehyde resin char and amorphous C-13 was investigated. The presence of O-containing complexes on the char surface was shown to increase the reactivity towards NO by providing free reactive sites. The presence of O-2 also enhanced the NO reduction reaction greatly. A mass balance during temperature-programmed reaction indicated nitrogen accumulation on the char surface. The reaction mechanism was examined by a transient kinetic approach, in which the reactant gas was switched from (NO)-N-14-O-16 to (NO)-N-15-O-18. The surface N-containing species produced and accumulated in the first stage, C(N-14), was found to react with (NO)-N-15-O-18, yielding (NN)-N-14-N-15. This indicated that one N-2 formation path is the reaction between nitrogen-containing surface compound and NO.