Equilibrium geometries have been optimized and harmonic vibrational frequencies calculated for HOOOCl using quadratic configuration interaction and coupled-cluster methods with Dunning’s correlation-consistent basis sets. There are two conformers of HOOOCl (a cis and a trans structure) that are very close in energy and separated by a barrier of 3.28 kcal mol⁻¹. In experiments, these two forms may co-exist. Rotational constants that can be used to experimentally characterize and identify HOOOCl are presented. The electronic absorption spectrum for trans-HOOOCl is calculated using multi-reference configuration interaction methods in order to support the spectroscopic search. At 5.84 eV a remarkably strong transition is computed corresponding to a σ(Cl–O′) → σ*(Cl–O′)-excitation. In addition, possible photodissociation pathways along the Cl–O′ and O′–O bonds for trans-HOOOCl have been examined. Potential photochemistry of HOOOCl is suggested for the first time.