The reaction of superoxide radical anions with sulfide radical cation complexes represents an important sulfoxide-forming process. Here, absolute rate constants for the reaction of sulfur-sulfur [>S...S <](+) and sulfur-nitrogen ([>S.. NH2R](+)) three-electron bonded sulfide radical cation complexes with superoxide and, for comparison, carbon dioxide radical anion have been measured by pulse radiolysis. For two different sulfur-sulfur bonded species, the intermolecular complex from dimethyl sulfide and the intramolecular complex from 1,5-dithia-3-hydroxycyclooctane, the rate constants for the reaction with superoxide are on the order of 1.6 x 10(10) M-1 s(-1) and with the carbon dioxide radical anion on the order of (6.5 +/- 0.2) x 10(9) M-1 s(-1). The fact that the stronger reducing carbon dioxide radical anion shows the lower rate constant can be rationalized by the higher internal reorganization energy of (CO2-)-C-. as compared to O-2(.-). The rare constant for the reaction of superoxide with the sulfur-nitrogen bonded radical cation of Met-Gly, k = 5.3 x 109 M-1 s(-1), is 3-fold lower as compared to that of the reaction with the sulfur-sulfur bonded radical cation complexes.