The mechanism of luminol chemiluminescence is a special case of nucleophilic addition to carbonyl compounds. The breakdown of the key intermediate, an alpha hydroxy hydroperoxide, produces a peracid ortho to an acyl diazene group. After intramolecular addition of the peracid, the energy from nitrogen expulsion is utilized in the formation of an anti-aromatic endoperoxide. Rupture along the O,O bond leaves a substantial part of the ensuing phthalate in its excited state. The emitter is shown to be a mono-protonated phthalate unaccessible by photoexcitation. The dark reaction is a concerted decomposion of the alpha hydroxy hydroperodixe to yield ground-state phthalate.