Iron-sulfur clusters are essential protein prosthetic groups that provide their redox potential to several different metabolic pathways. Formation of iron-sulfur clusters is assisted by a specialised machine that comprises, among other proteins, a ferredoxin. As a first step to elucidate the precise role of this protein in cluster assembly, we have studied the factors governing the stability and the dynamic properties of E. coli ferredoxin using different spectroscopic techniques. The cluster-loaded protein is monomeric and well structured with a flexible C-terminus but is highly oxygen sensitive so that it readily loses the cluster leading to an irreversible unfolding under aerobic conditions. This process is slowed down by reducing conditions and high ionic strengths. NMR relaxation experiments on the cluster-loaded protein also show that, once the cluster is in place, the protein forms a globular and relatively rigid domain. These data indicate that the presence of the iron-sulfur cluster is the switch between a functional and a non-functional state.