A self-assembled mixed-monolayer of 3-mercaptopropionic acid (MPA) and hexadecanethiol (KDT) was prepared on a gold electrode substrate by the co-adsorption from a mixed ethanol solution of these thiols, and the electrochemical reductive desorption behavior of the resulting mixed monolayer was studied in 0.5 M KOH aqueous solution. If ethanol containing 1 mM MPA and 0.2 mM HDT was used for preparation of monolayers, two cathodic waves due to desorption of the thiolates appeared at different potentials which accorded with reductive desorption potential of each component, suggesting strongly that a phase segregation occurred in the mixed monolayer. The integration of each current peak indicated that the fraction of HDT in the mixed monolayer tended to increase with increasing the soaking time of the gold electrode in the mixed solutions of thiols. The selective desorption of MPA was accomplished successfully from the mixed monolayer of MPA and HDT, and the resulting electrode showed electrochemical behavior characteristics of an array of microelectrodes, suggesting that domains of MPA were dispersed in the original mixed monolayer. By analyzing voltammograms taken in a Ru(NH3)(6)(3+) solution, assuming that a hexagonally arranged array of circular microdisk electrodes was formed by the desorption, the average radius of a domain of MPA was estimated to be ca. 25 nm for the mixed monolayer containing MPA with its molar fraction of 0.06.