The (33)S hyperfine structure has been observed and analyzed in the electron spin resonance of gamma-irradiated single crystals of cystine dihydrochloride at 77 degrees K and at room temperature. The radicals observed at room temperature and previously assigned as RCH(2)S are proved to be disulfide radicals, RCH(2)SS. However, the RCH(2)S radical is identified by the (33)S measurements in samples irradiated at 77 degrees K, annealed at 200 degrees K, and observed at 77 degrees K. This monosulfide radical appears to undergo reactions to form disulfide radicals at room temperature.