Determination of the ground- and excited-state unimolecular decomposition mechanisms of 1,2-dioxetanedione gives a level of insight into bimolecular decomposition reactions of this kind for Which some experimental results are reported. Although a few studies have put some effort to describe a biradical mechanism of this decomposition, there is still no systematic study that proves an existence of a biradical character. In the present study, state-of-the-art high-level multistate multiconfigurational reference second-order perturbation theory calculations are performed to describe the reaction mechanism of 1,2-dioxetanedione in detail. The calculations indicate that the decomposition of this four-membered ring peroxide containing two carbonyl carbon atoms occurs in concerted but not simultaneous fashion, so-called "merged", contrary to the case of unimolecular 1,2-dioxetane and 1,2-dioxetanone decompositions where biradical reaction pathways have-been calculated. At the TS of the ground-state surface, the system enters an entropic trapping region, where four singlet and four triplet manifolds are degenerated, which can lead to the formation of triplet and singlet excited biradical species. However, these excited species have to overcome a second activation barrier for C-C bond cleavage for excited product formation, whereas the ground-state energy surface possesses only one TS. Thus our calculations indicate that the unimolecular decomposition of 1,2-dioxetanedione should not lead to efficient excited-state formation, in agreement with the lack of direct emission from the peroxyoxalate reaction.