Biferrocene-modified Au clusters, comprising a 2.2 +/- 0.3 nm-diameter Au core covered with 20 biferrocene-terminated thiolates and 75 octyl thiolates on average, were synthesized by a substitution reaction of octanethiol-modified clusters with biferrocene-terminated alkanethiol, (eta(5)-C5H5)Fe(C10H8)Fe(eta(5)-C5H4CO(CH2)(7)SH) (1). The biferrocene-modified cluster undergoes two-step oxidation reactions in NBu4ClO4 + CH2Cl2 and the second oxidation process leads to the formation of a uniform redox-active Au cluster film on electrode. The surface plasmon absorption of the cluster film depends on the oxidation state of the redox active species on the cluster surface.