Two-component self-assembled monolayers composed of 3-mercaptopropionic acid (MPA) and 1-decanethiol (CH3(HC2)(9)SH:C10SH) on Au(111) were investigated with in situ scanning tunneling microscopy (STM) and cyclic voltammetry, where the monolayers I and II were prepared in 0.1 mM ethanol + water solutions with the ratios of MPA:C10SK = 95:5 and 90:10, respectively. In situ STM images revealed that both monolayer I and II consisted of phase-separated domains with molecularly ordered structures, each of which was predominantly formed by one of the constituent molecules. Based on the STM images, the surface fractions of constituent molecules were evaluated as MPA:C10SH=0.61:0.28 for monolayer I, and MPA:C10SH = 0.34:0.57 for monoiayer II. Cyclic voltammograms of the reductive desorption of the thiols provided the surface fractions of MPA:C10SH = 0.69:0.31 for monolayer I, and MPA:C10SH = 0.41 and 0.59 for monolayer II, which were in good agreement with the results obtained from STM images. Molecular resolution imaging allowed us to visualize the individual MPA and C10SH molecules and to determine the interfacial structures in the molecularly ordered domains of the phase-separated monoIayer. The C10SH domains in the monoIayer exhibited ordered phases with densely packed (root3 x root3)R30 degrees and p(3 x 2 root 3R-30 degrees) structures, which are well-characterized structures for alkanethiol monolayers. On the other hand, a completely different molecular arrangement of MPA defined as a (3 x 3) structure was consistently observed in the MPA domains, where the molecular arrangement is almost the same as that of (root3 x root3)R30 degrees but intermolecular hydrogen bending is thought to exist in the three neighboring MPA molecules located around the corner of the (3 x 3) unit cell. It was demonstrated that individual molecules of the monolayer constituents were successfully imaged in the two-component, phase-separated monolayer in solution.