The interactions among adsorbed species on metal surfaces have been studied for many years to understand their effects on adsorbate structures and chemical reactions on catalyst surfaces, on surfaces during microelectronics fabrication, on chemical sensors and electrodes, and on surfaces undergoing corrosion. It is no surprise, then, that the study of these interactions continues to attract attention, as the industrial need for control over these surfaces increases. For example, environmental protection demands new catalysts with ever increasing selectivity. In this field, one of the objectives is to establish an atomicscale description of the interactions between adsorbates to help guide the design of functional surface properties. Adsorbate structures have been analyzed previously with a variety of methods [1]. Even the adsorption of a single component frequently yields many different surface structures, depending on its coverage and the substrate temperature (which influences the adsorbate’s mobility and, thus, its ability to produce more-orless ordered structures and complete surface homogeneity). Therefore, separate domains are sometimes observed at low temperatures, where small or large domains with different local structures of the same adsorbate coexist. Their boundaries can be stabilized by antiphase periodicity, rotated structures or differently oriented adsorbates. Co-adsorption of two or more kinds of adsorbed species yields even more complex structures. Nevertheless, the co-adsorption of simple molecules generally has been classified simply into two groups depending on the magnitude of the interactions between adsorbates, i.e., separate domains and mixed phases. The former is likely when the repulsive interaction between different kinds of adsorbed species is stronger (or the attraction is weaker) than that between the same kinds of adspecies. Such coadsorption structures were found in many cases, first by low-energy electron diffraction [2] and photoemission electron microscopy [3], and, more recently, by scanning tunneling microscopy (STM) [4]. The segregation of adsorbates into separate domains of the different adsorbates was also proposed based on kinetic studies of chemical reactions combined with vibrational spectroscopy [5] or isotope tracing [6]. The