The electrochemical properties of 7-ferrocenyl(carbonyloxy)-1-heptanethiol [FcCO(2)(CH2)(7)SH] were characterized by an electrochemical quartz crystal microbalance (EQCM). A gold surface was modified with FcCO(2)(CH2)(7)SH by the self-assembly method. EQCM was used to investigate mass change during the redox reactions of FcCO(2)(CH2)(7)SH monolayers on a modified gold electrode in solution containing K3Fe(CN)(6) redox species. The FcCO(2)(CH2)(7)SH monolayer on gold acts as a mediator for the electron transfer between a gold electrode and electroactive species Fe(CN)(6)(3-/4-) in solution. In both cases, electrochemical current responses were complicated because the observed currents were due to the redox of both the ferrocenyl group immobilized on gold and others in electrolyte solutions. The interfacial mass change (i.e. frequency change) on the gold electrode surface was observed during the redox of ferrocenyl groups.