Tungsten nitride (W 2 N) was synthesized from WO 3 and NH 3 by means of temperature-programmed nitridation, and its activities for NO dissociation as well as NO reduction with H 2 were evaluated. In a 5% H 2 /1% NO/He stream at 400°C, stable NO conversion to N 2 of ca. 82% was observed within a test period of 40h. When the H 2 /NO ratio was reduced to 3/1, the conversion was ca. 80% and the catalyst started to deactivate after 28h of on-steam time. At a H 2 /NO ratio of 1/1, NO conversion to N 2 was ca. 63% and such performance could last for only 23h; even in the absence of H 2 , a conversion of ca. 52% lasted for 22h. We applied the XRD and XPS techniques, respectively, to examine the structural and compositional changes of W 2 N during the reactions. The results indicated that the gradual loss in catalytic activity was due to the bulk oxidation of W 2 N into WO 3 . Combined the activity measurements with those of NO adsorption studies, we deduce that W 2 N is catalytically active for NO adsorption and dissociation, and a H 2 /NO ratio of 5/1 is required to avoid heavy accumulation of surface oxygen and deactivation of the catalyst.