AbstractThe pyrolyisis of di‐tert‐butyl peroxyoxalate in the presence of para‐substituted benzaldehydes produces almost quantitatively the corresponding p,p′‐disubstituted benzils. The formation of these products is accompanied by chemiluminescence arising from excited triplets. From the quantum yield of excited triplet generation and the rate constants for the triplet photocleavage it is possible to obtain the change in Gibbs free energy associated with triplet formation. The values obtained are −5.6, −5.7 and −8.1 kcal/mol for benzil,p,p′‐dimethylbenzil andp,p′‐dimethoxybenzil, respectively.The pyrolysis of di‐tert‐butyl peroxyoxalate in the presence of isopropanol or benzoin leads to the formation of acetone and benzil. These products are generated in disproportionation processes involving the α‐hydroxy radical produced by hydrogen abstraction. The luminescence observed in these reactions constitutes the first experimental indication of excited species generation in the disproportionation