Binary self-assembled monolayers (binary SAMs) composed of 3-mercaptopropionic acid (MPA) and another alkanethiol which is longer than MPA were prepared by coadsorption on an Au-coated mica substrate (Au \ mica). The long alkanethiols chosen were 1-hexadecanethiol (HDT), 1-dodecanethiol (DDT), 1-decanethioI (DT), and 1-octanethiol (OT). Electrochemical measurements using the binary SAM-coated Au \ mica as a working electrode revealed that selective desorption of MPA from the prepared binary SAMs can be induced by polarizing the electrodes at -0.6 V vs. Ag \ AgCl in 0.5 mol dm(-3) KOH aqueous solution. STM observation of the electrode surfaces after the selective desorption of MPA gave clear images showing spatial distribution of the long thiols in the binary SAMs. In the cases of the binary SAMs of OT-MPA and DT-MPA, it was found that the thiols of inferior molar fraction tended to make separated round domains. However, as the chain length of the long thiol in the binary SAMs became longer, the long thiol tended to make separated domains regardless of its molar fraction, resulting in deviation of MPA domains from the round shape.