The adsorption of acetone on the Gc(100)-2 x 1 surface has been investigated using multiple internal reflection infrared spectroscopy and density functional theory quantum chemistry calculations and was found to be under thermodynamic control at room temperature. Acetone undergoes attachment at the carbonyl oxygen with loss of an a hydrogen at room temperature on the Gc(100)-2 x 1 surface to form an enol-like adduct. This so-called ene reaction has not been seen on the Si(100) surface, on which the reaction of acetone is under kinetic control and instead a less thermodynamically favored [2 + 2] addition across the carbonyl bond is observed. At low temperature, acetone adsorbs via dative-bond formation with the surface, which we find to be too weak, to remain at room temperature in contrast to the stronger dative bonds formed by amines on the Si(100) and Gc(100) surfaces. There is evidence that the acetone ene product may undergo oxygen migration and insertion into Ge-Ge bonds, especially at elevated surface temperatures. Whereas the reactions of organics on Si(100)-2 x 1 are typically under kinetic control, thermodynamic control on the Ge(100)2 x 1 surface is made possible by the reversibility of weak Surface adsorption. The strategic use of thermodynamically controlled reactions on the Ge(100)-2 x I surface represents a new and potentially powerful approach for the selective formation of organic-function ali zed semiconductor surfaces.