is unique in forming a 2D crystalline compound at its surface, with a fixed atomic-scale thickness, over a range of temperature. While this layer has not been imaged directly, diffraction studies have established that an ordered bilayer (the low-temperature (LT) phase) exists up to 12 °C above the eutectic temperature. [2,8] (A higher temperature phase has also been detected, although its properties are less clear. [9,10] ) At present there is no straightforward explanation for this unique behavior. Ordered surface phases have been explained by surface prefreezing, [11] a phenomenon that is analogous to the wellknown surface premelting. However, an ordered prefreezing layer is expected to be present only very near the transition temperature and to have a thickness that diverges as the temperature approaches the transition temperature, both of which are inconsistent with the behavior of Au-Si. The ordered surface phase in Au-Si also cannot be explained as a solute coming out of solution and wetting the surface, as can occur in dilute Ga solutions. [6] Here, we use in situ transmission electron microscopy (TEM) to observe directly the formation of a crystalline 2D compound at the surface of liquid Au-Si. By varying both temperature and composition we find that the 2D phase can be stable over a surprisingly large range, over 150 °C, extending above and below the eutectic temperature. We develop a simple thermodynamic model that explains the wide stability range and composition dependence. Given the persistence of the surface ordering, we explore its effects on critical aspects of the behavior of Au-Si. We find that the surface layer plays a key role in the pathway of eutectic decomposition of Au-Si into solid Au + solid Si on cooling, and also that it is the root cause of the dramatic changes observed in the catalytic properties of Au-Si on cooling. We derive a strategy for controlling the presence of the surface phase as a tool in nanostructure fabrication.Surface ordering on droplets of Au-Si of different dia meters is shown in Figure 1a,b and Movie S1 in the Supporting Information. These high spatial and temporal resolution data (see the Supporting Information) were recorded by in situ heating of Si substrates decorated with Au nanoparticles, with or without exposure to the Si source gas disilane (Si 2 H 6 ). On reaching the Au-Si eutectic temperature T E = 363 °C, [12] the Au reacts with Si to form Au-Si liquid droplets. Imaging the surfaces of these droplets in profile view (Figure 1a) shows that there are two well-defined crystalline layers at the surface. The ordering can also be detected in projection over the entire droplet area in Figure 1a. In the smaller droplet shownIn situ transmission electron microscopy reveals that an atomically thin crystalline phase at the surface of liquid Au-Si is stable over an unexpectedly wide range of conditions. By measuring the surface structure as a function of liquid temperature and composition, a simple thermodynamic model is developed to explain the stability of the or...