The ENDOR signals from the CH 2 protons of the disulfide anion and of the disulfide cation produced in a single crystal of l-cystine dihydrochloride γ-irradiated at 77 K have been detected and the hyperfine tensors determined. The principal values of the hyperfine tensors for the two nonequivalent protons are found to be (33.1, 27.6, 25.9) MHz and (24.0, 18.9, 17.3) MHz for the anion, (21.3, 17.5, 16.5) MHz and (14.3, 5.5, 4.1) MHz for the cation. UHF INDO calculations have been performed, which predict that the unpaired electron of the anion occupies the antibonding sigma-type orbital of the disulfide bond, whereas that of the cation occupies the antiboding π-type orbitals, both constructed dominantly from the sulfur 3 p orbitals. The UHF INDO results combined with the point dipole approximation for the dipolar hyperfine tensors have given assignments of the ENDOR signals to specific protons of the CH 2 groups. The isotropic parts of the hyperfine tensors for the CH 2 protons are interpreted by hyperconjugation mechanism, i.e., by the relation a = B cos 2 φ, where B is determined to be 25.9 and 36.8 G for the anion and the cation, respectively.