There has recently been considerable interest in the reaction between Co and a clean Si surface. This interest stems from the epitaxy of CoSi2 and NiSi2 on Si and its potential for the construction of reliable and stable metal-semiconductor structures. In fact, the fabrication of a Si/CoSi2/Si transistor has been recently reported.[l] On a more fundamental side, it has been possible to address the problem of the relation between Schottky barrier height and structure at the NiSi2/Ni interface, which exhibits both a rotated (B-type) and unrotated (A-type) geometry. For CoSi2/Si only the 180° rotated, B-type disilicide is formed. By studying the room temperature interface, we have attempted to describe the nature and physical extent of reaction products; such knowledge is important to understand the formation of interface silicides which ultimately control the nature of the high temperature epitaxial interface.