The electrochemical behaviour of the monosubstituted cyclohexadienyl complexes [Fe( η 5-2-R-cyclohexadienyl)(CO) 3][PF 6] (R H, Me, n-Bu, MeO, Ph, MeOC 6H 4, CF 3C 6H 4), [Fe( η 5-1-EtO-cyclohexadienyl)(CO) 3][PF 6], the disubstituted species [Fe( η 5-R,R′-cyclohexadienyl)(CO) 3][PF 6] (R R′ MeO or Me; R Me, R′ MeO), and [Fe( η 5-R-cyclohexadienyl)(CO) 2(PPh 3)][PF 6] (R H, Ph), was studied by cyclic voltammetry and controlled potential electrolysis. They undergo a single electron irreversible cathodic process at a potential which is dependent on the electronic properties of both the cyclohexadienyl substituent and the co-ligands at the metal. In particular, tricarbonyliron complexes with aromatic substituents are reduced at significantly higher potentials than those with aliphatic ones, whereas substitution of a carbonyl by a triphenylphosphine ligand lowers the cathodic potential. By cathodic controlled potential electrolysis, they undergo CC coupling and, in some cases (unsubstituted or alkoxy-substituted cyclohexadienyl complexes) also FeFe coupling (with loss of CO) to form the corresponding dimeric species. The formation of these products has also been detected, in a number of cases, in the time-scale of the cyclic voltammetry.