The Fe(CN)3-(6) oxidation of the crystallographically characterized [[3Fe-3S], [4Fe-4S]] ferredoxin I of Azotobacter vinelandii has been studied using absorption, circular dichroism, magnetic circular dichroism, and EPR spectroscopies. A paramagnetic intermediate is observed en route to Fe-S cluster-free apoprotein, possessing an anisotropic g approximately equal to 2 EPR signal, surviving to temperatures greater than 77 K. This species is shown to result from 3-electron oxidation of the [4Fe-4S] cluster, without modification of the [3Fe-3S] cluster. However, it does not give rise to observable paramagnetic magnetic circular dichroism in the visible-near UV spectral region and is therefore neither an oxidized HIPIP [4Fe-4S] cluster nor an oxidized [3Fe-3S] cluster. We identify the paramagnetic species as a cysteinyldisulfide radical formed on dissociation of an oxidized cysteinate and an oxidized sulfide ion from the [4Fe-4S] cluster. This conclusion is consistent with the observed reaction stoichiometry, the spectroscopic results obtained, known EPR spectra of disulfide radicals, and the reconstitution of the native [4Fe-4S] cluster by dithiothreitol alone. This reaction, earlier interpreted as a HIPIP-type oxidation, is a previously uncharacterized oxidation reaction of [4Fe-4S] clusters.