Unusually strong apparent surface activity of the pair of 2-mercaptoethane sulfonic acid (MES) and 2-aminoethanethiol (AET) in forming a phase-separated ternary self-assembled monolayer (SAM) composed of MES, AET, and 1-dodecanethiol (DDeT) has been investigated with changing the conditions for preparing the SAM by coadsorption of the thiols from an ethanolic solution. When the total concentration of the thiols, c(total), is 1 x 10(-3) mol dm(-3) or higher, the MES-AET pair forms electrostatically stabilized domains from an early stage of the adsorption after the immersion of a gold substrate in the bathing ethanol solution. The total area of the MES-AET domains does not change with time for longer immersion. The adsorption of DDeT is significantly weaker than the MES-AET pair under such conditions, despite the fact that the intrinsic surface activity of DDeT is greater than those of MES and AET. DDeT-rich islands of typically a few nanometer across spread in the SAM and the size and the distribution of the islands are invariant over a week. In contrast, when c(total) is lower than 0.1 x 10(-3) mol dm(-3), the adsorptivity of DDeT is stronger than that of the MES-AET pair, and MES and AET initially adsorbed are gradually replaced with DDeT at longer immersion time. This switching of the mode of phase separation with the change in c(total) suggests the crucial roles of the electrostatic interaction of MES and AET in the course of the formation of the SAM and of the hydrophilic barrier formed on the domains to prevent the replacement by hydrophobic DDeT molecules. (c) 2006 Elsevier B.V. All rights reserved.