The interactions of the proteins bovine insulin and bovine serum albumin with the water-soluble cluster [Fe4S4(SCH2CH2CO2)4]6–(1) are described. The proteins were found to stabilise the cluster to air. Changes in the visible absorption and circular dichroism spectra indicated that inclusion of the cluster into the protein had occurred. Anodic shifts of redox potential between 170 and 220 mV were observed for the included clusters using cyclic voltammetry. The redox potentials of the bovine serum albumin iron–sulphur cluster exhibited a pH dependence of –63 mV per pH unit (one proton dependence) in the pH range 5–8. Beyond pH 8, a pH dependence of up to –235 mV per pH unit was observed in accordance with four reversible thiolate–hydroxide species exchange reactions.