Treatment of Clostridium pasteurianum ferredoxin (CpFd) with stoichiometric amounts of potassium ferricyanide results in the specific conversion of cluster I into a Fe3S4+ species while leaving cluster II unaltered. Ferricyanide-treated CpFd derivative has been purified and characterized through biochemical and spectroscopical techniques. The cluster conversion process is reversible and reconstitution of native CpFd has been afforded under appropriate conditions.