A novel, acidic photo-oxidation product of benzene has been obtained, for which tentatively the structure: o-formyl-vpyrane (I) is proposed. Preparation: An aerated solution of benzene in water (5 1, pH = 5, 1 M phosphate buffer*) was irradiated by a submerged Philips TUV 30 W low-pressure mercury lamp, while being cooled with water. A circulating benzene phase extracted I from the irradiated aqueous solution and transferred it to 2 N aqueous sodium hydroxide (150 ml). Percolation with ether of the allcaline extract at pH = 11 removed phenol (1,2) and neutral contaminants; subsequent percolation at pH = 6 afforded I in dilute ethereal solution, the residue of which upon repeated high-vacuum sublimation yielded I in the form of colourless needles, which melted at -25 'C while decomposing. Although I is formed with a spectrophotometrically estimated quantum yield of 5-lo%, only loo-150 mg could be isolated after 24 hours of irradiation -i.e. about 0.1% overall quantum yield. The properties of I determined so far are given below. It should be emphasized, however, that the solutions decompose rapidly** which was very troublesome, especially in the NMR and IR analyses. UV absorption spectra. Besides end absorption below 2200 fi (0.1 mmol/l) one sharp band prevails: In water at pH ~7, A,, = 2940 2 (E = 12,500), extractable with organic solvents (e:g. cyclohexane, A max = 2900 8); at pH > 12 Amax = 3080 % ( E = 25,000). not extractable. Similar pH-dependent UV spectra were obtained upon irradiation of oxygenated 1 : 1 aqueous-ethanolic solutions of toluene, o-and m-xylene, tert-butylbenzene, chlorobenzene and diphenyloxide. NMR and IR spectra indicate the absence of carboxylic acid and of hydroxyl groups. The following NMR signals may be attributed to I (Ccl4 solution, 6 in ppm relative to TMS, all single lines):