The oxidation of carbon monoxide by ClO radicals was studied by ab initio molecular orbital theory calculations. Geometry optimizations and vibrational frequencies were computed using two methods: Moller-Plesset second-order perturbation theory (MP2), and quadratic configuration interaction in the space of single and double excitations (QCISD). Single-point energy calculations were performed at the QCISD level with triple excitations treated perturbatively (QCISD(T)) and the aug-cc-pVTZ basis set. Canonical transition state theory was used to predict the rate constants as a function of temperature (550-2500 K), and three-parameter Arrhenius expressions were obtained by fitting to the computed rate constants. The possible impact of the title reaction in combustion chemistry is also discussed.