A mechanistic study of the reaction Cl + O-3 -> ClO + O-2 at the high level of multi-reference DET-MRPT2 theory for both the static reaction path and the dynamics is presented. It is found that a spin-flip takes place along the computed dynamical path, a point neglected in previous studies. The time scale of the spin-flip is estimated from the dynamics. The algorithmic improvements that make possible the high-level multi-reference dynamics simulation are briefly discussed. (C) 2012 Elsevier B. V. All rights reserved.