Cysteinyl radicals have been studied by ESR spectroscopy after generation by irradiation of single crystals1and frozen matrices2. The spectra reported were broad lines, and had been assigned to various radical species with high electron spin density is concentrated in a nonbonding 3p orbital of the sulfur atom, accounted for the basic features3. However, it had not been established whether such spectra also would be produced by cysteinyl radicals when present in solution. As it is probable that cysteinyl radicals nay also be formed, among other species, in certain systems presently under study in this laboratory, it was necessary to determine the spectroscopic characteristics of such free radicals when produced in the liquid phase.