The gas-phase reaction of activation of C-C double bond of C2H4 by VO2+ has been investigated using density functional theory (DFT) at the UB3LYP/6-311G(2d,p) level. The calculation results show that the reaction experiences rearranged process. On the basis of Hammond postulate, this is a typical 'two-state reactivity' (TSR) reaction. The involving crossing between the singlet and triplet potential energy surfaces has been discussed by means of the intrinsic reaction coordinate (IRC) approach used by Yoshizawa et al., and a minimum energy crossing point (MECP) that we obtained by the mathematical algorithm proposed by Harvey et al. has been also employed. (C) 2008 Published by Elsevier B.V.