The presence of disulfide bonds can be detected unambiguously only by X-ray crystallography, and otherwise must be inferred by chemical methods. In this study we demonstrate that 13C NMR chemical shifts are diagnostic of disulfide bond formation, and can discriminate between cysteine in the reduced (free) and oxidized (disulfide bonded) state. A database of cysteine 13C C(alpha) and C(beta) chemical shifts was constructed from the BMRB and Sheffield databases, and published journals. Statistical analysis indicated that the C(beta) shift is extremely sensitive to the redox state, and can predict the disulfide-bonded state. Further, chemical shifts in both states occupy distinct clusters as a function of secondary structure in the C(alpha)/C(beta) chemical shift map. On the basis of these results, we provide simple ground rules for predicting the redox state of cysteines; these rules could be used effectively in NMR structure determination, predicting new folds, and in protein folding studies.