FTIR spectroscopy has been used to study the generation of radicals at low temperature upon UV irradiation of ubiquinones, naphthoquinones, and plastoquinones. Radical formation was ascertained by EPR spectroscopy. The effects of solvent, temperature, and chemical nature of the chain substituent on the radical photogeneration were studied. The presence of a side chain bearing at least one isoprene unit plays an essential role in the radical generation process. O-18 labeling on both carbonyls and specific C-13 labeling on each carbonyl group were used to investigate the nature of the radicals. UV irradiation at low temperature of quinones bearing an unsaturated side chain leads to the formation of the protonated semiquinone radical. On the basis of FTIR and Raman data, the C - O stretching mode of the protonated semiquinone radical of ubiquinone, naphthoquinone, and plastoquinone is observed at approximate to 1518, 1536, and 1504 cm(-1), respectively. An intramolecular hydrogen atom transfer between the first isoprene unit of the side chain and the proximal carbonyl of the quinone ring is demonstrated, strongly favoring a diradical formation mechanism previously proposed to occur upon UV irradiation of plastoquinone at room temperature (Creed, D.; Hales, B. J.; Porter, G. Proc. R. Sec. London, A 1973, 334, 505).