The mechanism of the gas-phase reaction N2O + CO -> N-2 + CO2 has been elucidated by means of various density functional methods in conjunction with the ab initio methods. An indirect and step-wise pathway in which CO approaches to the N-end of N2O, is concluded to be lower in energy than an O-end route, which is a direct O-atom transfer pathway initiated by CO approaching to the O-end of N2O. High level ab initio method (i.e. QCISD(T)//QCISD) suggests that MP2 may inappropriately estimate the electron correlation effect, leading to unreasonable geometry and energetics for this reaction. (C) 2009 Published by Elsevier B.V.