Thioredoxin contains a single disulfide bond that can be reduced without perturbing significantly the structure of the enzyme. Upon reduction of the disulfide, protein stability decreases. We have experimentally tested the expected linkage relationship between disulfide bond formation and protein stability for thioredoxin. In order to do this, it is necessary to measure the equilibrium constant for disulfide bond formation in both the folded and unfolded states of the protein. Using glutathione as a reference species, we have measured the equilibrium constant for forming the disulfide bond (effective concentration) in thioredoxin as a function of urea concentration. As a control, we show that urea per se does not interfere with our measurements of thiol-disulfide equilibrium constants. Comparison of the values obtained for disulfide bond formation in the folded and unfolded states with the free energies for unfolding oxidized and reduced thioredoxin using circular dichroism confirms the expected linkage relationship. The urea dependence of thiol-disulfide equilibria provides a sensitive assay for folded structure in peptides or proteins. The method should also be useful to evaluate the stabilizing or destabilizing effect of natural or genetically engineered disulfides in proteins. In future work, the effects of amino acid substitutions on disulfide bond formation could be evaluated individually in the native and unfolded states of a protein.