The mechanisms of the reaction between N2O and CO catalyzed by MO+ (M = Ru, Os) have been investigated using the density functional theory (DFT). The potential energy surfaces, the crossing points and the corresponding minimum energy crossing points have been explored. The probability of hopping in the vicinity of the crossing has been calculated by the Landau-Zener-type model. The results indicate the reactions are O-atom abstraction mechanism for the catalysts RuO+ and OsO+. The large probability value shows the intersystem crossing occurs efficiently, which makes the reaction access to a lower energy pathway and accelerate the reaction rate. (C) 2010 Elsevier B.V. All rights reserved.