The mechanism of the cyclic reaction N2O ＋CO →N2 ＋CO2 catalyzed by Ir＋ has been investigated on both triplet and quintet potential energy surfaces (PES). The reactions were studied by the UB3LYP density functional method and the Stuttgart pseudopotentials. The O-atom affinities of the correlative species were calculated, which testified that the cyclic, catalytic function of Ir＋ was thermodynamically allowed. Moreover, the calculated results of different spin PES show that the reaction proceeds in a two-step manner and spin crossing between different PES occurs. The involved crossing between the PES has been discussed by means of the intrinsic reaction coordinate approach used by Yoshizawa et al., and the crossing points were located. Furthermore, both steps of the reaction are exothermic and the overall exothermicity is 358.9 kJ8226;mol－1.