Stein and Weiss1 have shown that hydroxyl radicals can be detected in solution by their ability to oxidize the benzene nucleus, leading to the formation of phenol derivatives. These hydroxyl radicals were produced in situ both by irradiation of aqueous substrate solutions with X-rays, α-rays, etc., and by the use of Fenton's reagent. Similar results were obtained with the latter by Merz and Waters2. Evans and Uri3 have shown that irradiation of aqueous solutions of ion pair complexes such as Fe3+OH−, Fe3+Cl−, Fe3+F−, etc., with ultra-violet light (300–400mµ) produces atoms or free radicals in solution able to initiate the polymerization of vinyl compounds. Experimental evidence indicates that the ion-pair complexes postulated by Rabinowitch and Stock-mayer4 are the photochemically active entities in ferric salt solutions. In the light of these findings, we investigated the effect of monochromatic light of wave-length 365 mµ, on aqueous solutions of benzoic acid in the presence of ferric ion-pair complexes. Benzoic acid was chosen as substrate because the formation of the very intensely coloured ferric salicylate complex is convenient for a photometric determination.