The thermal and photochemical O−O bond dissociation mechanisms in the aromatic oxygen carrier photosensitizer anthracene-9,10-endoperoxide have been elucidated using high-level multiconfigurational ab initio calculations (CASSCF and MS-CASPT2). Our results show that in both the thermal and the photochemical pathways, the system proceeds through a degeneracy of four singlet plus four triplet states. This high-order degeneracy provides an efficient funnel for radiationless deactivation from the lowest excited state, indicating that the system does not dissociate in the excited-state manifold but that the products are rather formed in the electronic ground state. Accordingly, the homolysis of the peroxide group leads to four plus four ground-state biradicals, which are the precursors of the experimentally observed rearrangement products.