An analytical potential energy surface of the lowest singlet (1)A' state for the O(D-1) + N2O --> NO + NO/N-2 + O-2 reaction has been developed on the basis of extensive ab initio electronic structure calculations at the CASPT2/cc-pVDZ level of theory within C-s constraint. A many-body expansion type function was employed to fit the calculated ab initio points. Classical trajectory calculations have been carried out using the newly developed potential energy surface. We found that the initial orientation angle significantly affects the NO + NO/N-2 + O-2 product branching and the branching ratio decreases as the relative translational energy increases. (C) 2002 Elsevier Science B.V. All rights reserved.