Classical trajectories have been integrated to study the O + ClO reaction, both reactive and vibrational energy transfer processes, for the range of temperatures 100 ≤ T/K ≤ 500 using momentum Gaussian binning. The employed potential energy surface is the recently proposed single-sheeted double many-body expansion potential energy surface for the 2 A″ ground-state of ClO 2 based on multireference ab initio data. A capture-type regime with a room-temperature rate constant of (17.8 ± 0.5) × 10 −12 cm 3 s −1 and temperature dependence of k(T/K)/cm 3 s −1 = 22.4 × 10 −12 × T −0.81 exp(−39.2/T) has been found. Although the value reported here is half of the experimental and recommended one, tentative explanations are given. Other dynamical attributes are also examined for the title reaction, with state-to-all and state-to-state vibrational relaxation and excitation rate constants reported for temperatures of relevance in stratospheric chemistry. 1. INTRODUCTION During the 1960s, improved laboratory measurements have shown that pure-oxygen reactions could not explain the abundances of atmospheric ozone in the upper-stratosphere through the so-called Chapman mechanism. Additional processes were required, bearing in mind that the number of constituents in that atmospheric region is limited. Bates and Nicolet 1 proposed that trace constituents could be the missing clue. However, because they are in trace concentrations, they would be rapidly consumed, and hence such a limitation could only be overcome if they could participate in a catalytic cycle capable of regenerating them at the end of the process. One possibility would be the catalytic cycle: