A H-2 NMR study has been performed on a 7Fe8S ferredoxin from Bacillus schlegelii which has been overexpressed in Escherichia coli. The protein cysteines have been deuterated at the beta position by incorporating labeled cysteines into the growth media. The protein contains an Fe3S4 and an Fe4S4 cluster. The former has been investigated in the [Fe3S4](0) and in the [Fe3S4](+) states. Whereas the [Fe3S4](+)-containing species provides sharp H-1 and H-2 NMR spectra for the signals of the cysteine ligands, no corresponding H-1 or H-2 signals have been detected from the [Fe3S4](0)-containing species. Theoretical considerations predict observability of these signals unless a chemical equilibrium is operative. It is proposed therefore that the Fe3+ ion and the two Fe2.5+ ions constituting the cluster exchange their valency with a rate in the order of 10(6) s(-1), which would cause coalescence of the signals.