The new highly accurate complete basis set model, CBS-QB3, was employed here to elucidate the long experimentally discussed problem in a general class of chemiluminescent reactions involving peroxyoxalate systems. Both the stability comparison and the vibrational spectra favor that the intermediate is better to be recognized as the cyclic singlet 1,2-dioxetanedione with the C-2v symmetry, which verifies the experimental suggestion yet provides more characterization information. Another two kinds of minimum species in its potential energy surface (PES) are two kinds of product: (1) two carbon dioxide and (2) two carbon monoxide and one oxygen, where the thermodynamic parameters correctly identify their relative yield in the experiment-the former is much more abundant than the latter. In a complete search of minimum states in its PES, the triplet C-2v and D-2h states were found, which is energetically unfavorable compared with the singlet C-2v state. Their vibrational data also support some experimental conclusions of ruling out a radical intermediate. In contrast, the singlet D-2h state was found to be a transition state for the "up" and "down" singlet C-2v states. The complete active space self-consistent-field calculations with the second-order Moller-Plesset correlation energy correction also support that the most stable species is the singlet C-2v state and the singlet D-2h state is more energetically favorable than its triplet counterpart.