Low-lying singlet and triplet electronic excited states of ClOOCl are presented. Calculations of the excitation energies and oscillator strengths are reported using excited state coupled cluster response methods, as well as the complete active space self-consistent field method with the full Breit-Pauli spin-orbit operator. These calculations predict that for ClOOCl there should be a weakly absorbing triplet state lying below the lowest absorbing singlet excited state. This state is predicted to have an absorption maximum at about 385+/-25 nm. This lowest triplet state is calculated to be dissociative and leads to ClOO+Cl. (C) 2004 American Institute of Physics.