AbstractThe 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 phth