An electrochemical quartz crystal microbalance (EQCM) was employed to investigate mass transport during the redox reaction of the ferrocenylundecanethiolate (FcC(11)S(H)) monolayer modified gold electrode in solution containing other redox species. The FcC(11)S-monolayer on gold acts as a barrier for the electron transfer between a gold electrode and Fe(CN)(6)(4-/3-) in solution and as a mediator for the reduction of Fe3+ 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 frequency change, i.e. interfacial mass change on the gold electrode surface, was observed only during the redox of ferrocenyl groups. The complex current response was deconvoluted into the current components of the redox reaction of ferrocene and that of other redox species in solution by comparing cyclic voltammograms with the current calculated from frequency changes.