The interfacial microstructure, based on a CH3(CH2)(15)SH and (HSCH2CH2)(2)O self-assembled mixed monolayer, can be controlled by electrochemical desorption at a gold electrode. The size, number and chemical characteristics of defect sites can be electrochemically reproduced. The electrochemically produced defect size at the thiol monolayer on gold was calculated by the change on the electron transfer kinetics. A simple model of steady state current in a single defect site, which involves (a) diffusion in the solution; (b) diffusion inside the defect and; (c) interface mass transfer, has been proposed. The defect size effect on the mass transport and the characteristic halfwave potential is presented. If the radius of the defect site is similar to the dimension of a probe molecule or interactions between the probe molecules and the inner surface of the defect site are strong enough to hold the probe molecules, the physical diffusion of the probe molecules decreases. Thus, the charge transfer of the probe molecule in the defect site might depend on electron hopping among probe molecules.