The potential energy surface (PES) of [HClO2] system has been investigated at second-order Moller-Plesset perturbation (MP2) and density functional theory levels using, respectively, the 6-311++G** and 6-311++G(2df,2pd) basis sets. The structural parameters of the isomers of [HClO2] were also optimized at complete active space [(CAS(12,12)] self-consistent field level using the 6-31G** basis set. Three isomers of relative thermodynamic stability, HOOCl (1) > HOClO (2) > HClO2 (3), have been identified as energy minima. Besides these covalently bound minima, various loose hydrogen-bonded complexes (OOH ... Cl 5a(s), 5'a(t), and O ... HOCl 7a(t)) have been located on the singlet and triplet PES. Isomerization, molecular elimination, and direct hydrogen abstraction saddle points have been traced and a qualitative understanding of the mechanism and kinetics of the stratospherically important HO + ClO and HO2 + Cl reactions has been derived.