Chapter 1 in this thesis provides a short introduction to the biochemistry and chemistry of iron-sulphur systems, particularly in relation to biological nitrogen fixation and the inhibition of nitrogenases and hydrogenases by carbon monoxide. Chapter 2 describes some results bearing on the interaction of the inhibitor CO with simple iron-sulphur and iron-sulphur-vanadium systems under reducing conditions. It is shown that in the presence of CO an Fe454-centre can be reduced to carbonylated {Fe252} and {Fe35}-fragments. The crystal structure of [Fe35(CO)g ][PPh4]2 is described. Chapter 3 describes infra-red/electrolysis studies of the [Fe4S4 (5Ph)4 ]2-/CO system and of related Fe-S species. Evidence is presented for the occurrence of [Fe454(CO)12]2- as an intermediate in the pathway of reduction of [Fe454 (5Ph)4 ]2- to [Fe252 (CO)612- under an atmosphere of co. Chapter 4 describes the fortuitous isolation of [V(52)2(terpyridine)] from a reaction of 2,2',2"-terpyridine with [VFe3S4Cl3 (dmf)]- and its rational synthesis from 2,2' ,2"-terpyridine and [V54 ]3-. Some physical and chemical and electrochemical properties of this compound are described together with a description of its X-ray crystallographic structure.