Electrochemical investigations of a tri-iron cluster, [Fe-3(mu(3)-S)(2)(CO)(9)] (designated as Fe3CO thereafter), which possesses a core of [(FeFeFeI)-Fe-II-Fe-I], is described. In 0.5 M [NBu4]BF4/dichloromethane, the cluster exhibits a fairly reversible redox process at -1.03 V and irreversible reduction wave at ca. -1.75 V. The latter is attributed to the reductions of both the decomposed product of the monoanion, [Fe3CO](center dot-), and the isomer of this anion. This cluster catalyses proton reduction with the presence of acid HBF4-Et2O in dichloromethane. Possible mechanisms were proposed to elucidate its electrochemistry and electrocatalytic behaviours on proton reduction. Digital simulations were performed to verify the proposed mechanisms and kinetic parameters were generated at our best estimation in the simulations. The simulated cyclic voltammograms fit well with the experimentally observed ones, which largely supports the proposed mechanisms. (C) 2009 Elsevier Ltd. All rights reserved.