Ceric ion oxidation of thiols in solution leads to a variety of free radical signals, ascribable to at least seven different species. Some of them, like the thiyl (RS·) free radical at g = 2.0106 ± 0.0002 have been conclusively identified, while others have been only tentatively assigned. One of these free radicals, arising upon the oxidation of mercaptoethanol, yields a 1:2:1 triplet signal with a C 1H of 4.0 gauss and 1.4 gauss line width. Evidence is presented on the nature of this species as the alkoxy (RO·) free radical formed in an intramolecular hydrogen transfer reaction. Other thiols, including 1,3-dimercaptopropanol-2, 1,4-dithioerythritol, and 1,4-dithiothreitol, examined in this system, exhibit the expected 1:1 doublet for the RO· free radical and support this assignment. ESR spectral characteristics of the several short lived free radical species generated from these and other hydroxy thiols are described and interpreted. The influence of conformational factors on the hydrogen transfer reaction and their significance in relation to the mechanisms of radioprotective action are discussed.