Self-assembled monolayers of thiols have been studied by electrochemical scanning tunneling microscopy (EC-STM) in solution, which successfully disclosed the interfacial structures of the monolayers on Au(111) electrodes. 3-mercaptopropionic acid (MPA) monolayers exhibited an ordered structure defined as (3×3), and it is expected that intermolecular hydrogen bonding exists between the carboxylic acid head groups of the monolayer. The ES-STM showed large and small phase boundaries of the MPA monolayers. A two-component self-assembled monolayer of MPA and decanethiol (C₁₀SH) exhibited phase-separated domain structures consisting of each component. It was also visualized that MPA and C₁₀SH domains have ordered structures of (3×3) and p(3×2√3R-30^o), respectively.