Electron spin resonance (EPR) spectra demonstrate the formation of a complex between the ascorbate ion and disulfides such as oxidized glutathione, cystine, gamma globulin, bovine serum albumin, 4-4′ dithiopyridine, and dithiobutyric acid. However, except in the case of 4-4′ dithiopyridine. ascorbic acid (AH2) is not capable of reducing the disulfide bond in these compounds. These results can be explained in terms of the redox potential of ascorbic acid and its radical semidehydroascorbic acid (SDA). These compounds can reduce the S-S bond of 4-4′ dithiopyridine, the redox potential of which is E′ = -0. 1 V and, thus, more positive than the ones determined for the ascorbic acid system.