Reactivity of gas phase zirconium oxide clusters (ZrnOm+) toward NO and CO gases was investigated by mass spectrometry in combination with post heating. Reaction of ZrnO2n+x+ with NO gas resulted in the depletion of extremely oxygen-deficient clusters and the formation of oxygen-rich clusters, ZrnO2n+x+ (0 <= x <= 3). Reaction with CO substantially lead to an increase in the amount of ZrnO2n-2+ and ZrnO2n-1+, clusters and depletion in the amount of Zr-n,O-2n(+). The catalytic cycle, achieved by regenerating ZrnO2n+ by the oxidation of ZrnO2n-2+ by NO, were discussed in comparison with the reactivity of cerium oxide clusters. (C) 2016 Elsevier B.V. All rights reserved.