Ab initio molecular orbital calculations have been carried out to determine the relative stabilities, and the spectroscopy of isomers of HClO2. Two straight-chain isomers, HOOCl and HOClO, and one branched species, HClO2, have been identified as energy minima on the HClO2 potential energy surface. The highest level of calculation performed (CCSD(T)/ANO4) suggests that HOOCl is the lowest energy form. The branched structure is 51.0 kcal mol(-1) higher in energy. The HOClO is 8.3 kcal mol(-1) higher in energy compared to the lowest energy structure. Intense infrared absorptions for HOOCl are calculated at 1399 and 392 cm(-1), at 1186 and 249 cm(-1) for HOClO, and 2168 and 1009 cm(-1) for HClO2. The heat of formation of HOOCl calculated using perturbation theory is similar to that determined using the CCSD(T) level of theory.