[carbonyl-14C]-4-Benzoylbenzoic acid, a photolabelling reagent, was synthesised with high specific activity in 57% radioactive yield from [carboxyl-14C]-benzoic acid. Radioactive synthesis allowed elucidation of the mechanism of a previous unlabelled synthesis using benzoic acid, methyl ester. An improvement in the experimental conditions led to a new synthesis of radioactive benzoylbenzoic derivatives with good