The reaction of CIO with HO₂ has been studied using ab initio molecular orbital theory with electron correlation being taken into account by Møller–Plesset (MP) perturbation theory. Saddle-point geometries, energies and harmonic vibrational wavenumbers have been calculated at the MP2/6-31G** level and barrier heights at the MP4/6-311G** level. The study shows that the products and mechanism depend on which potential-energy surface the reaction occurs. If the reaction proceeds along the triplet reaction surface, ground-state HOCl (¹A′) and ground-state O₂(³Σ_g^–) are formed. If the reaction takes place on the singlet reaction surface the same products are formed, but in this case the molecular oxygen is formed in a singlet state. The CIO + HO₂ reaction proceeds via a multi-step reaction mechanism on the singlet surface and via a direct hydrogen-abstraction mechanism on the triplet surface.