Avidin-biotin interaction at a biotin/thionine modified Au electrode was investigated with voltammetry. The electrode was modified with a self-assembled monolayer (SAM). The avidin-biotin interaction was monitored through change of the electrode response of thionine. The electrode response decreased with increasing concentration of the avidin. This is because the thionine moiety was held at the binding site due to the avidin-biotin interaction. Furthermore, the length of spacer between the thionine and biotin moieties, which influences the avidin-biotin binding, was examined.