Peroxyoxalate chemiluminescence is used in self-contained light sources, such as glow sticks, where oxidation of aromatic oxalate esters produces a high-energy intermediate (HEI) that excites fluorescence dyes via electron transfer chemistry, mimicking bioluminescence for efficient chemical energy-to-light conversion. The identity of the HEI and reasons for the efficiency of the peroxyoxalate reaction remain elusive. We present here unequivocal proof that the HEI of the peroxyoxalate system is a cyclic peroxidic carbon dioxide dimer, namely, 1,2-dioxetanedione. Oxalic peracids bearing a substituted phenyl group were unable to directly excite fluorescent dyes; hence, they could be ruled out as the HEI. However, base-catalyzed cyclization of these species results in bright chemiluminescence, with decay rates and chemiexcitation quantum yields that are influenced by the electronic phenylic substituent properties. Hammett (ρ = +2.2 ± 0.1) and Brønsted (β = -1.1 ± 0.1) constants for the cyclization step preceding chemiexcitation imply that the loss of the phenolate-leaving group and intramolecular nucleophilic attack of the percarboxylate anion occur in a concerted manner, generating 1,2-dioxetanedione as the unique outcome. The presence of better leaving groups influences the reaction mechanism, favoring the chemiluminescent reaction pathway over the nonemissive formation of aryl-1,2-dioxetanones.