The physico-chemical properties of passivated gamma -Mo2N have been investigated. The material showed high activities for NO direct decomposition: nearly 100% NO conversion and 95% N-2 selectivity were achieved at 450 degreesC. The amount of O-2 taken up by gamma -Mo2N increased with temperature rise and reached 3133.9 mu mol g(-l) at 450 degreesC; we conclude that formation of Mo2OxNy occurred. This oxygen saturated gamma -Mo2N material was catalytically active: NO conversion and N-2 selectivity were 89 and 92% at 450 degreesC. We found that by means of H-2 reduction at 450 degreesC, Mo2OxNy could be reduced back to gamma -Mo2N and the oxidation/reduction cycle is repealable: such a behaviour and the high oxygen capacity (3133.9 mu mol g(-l)) of gamma -Mo2N suggest that gamma -Mo2N is a promising catalytic material for automobile exhaust purification.