Thermal desorption methods have been used to investigate the interaction of propylene (C3H6) with Si(100)‐(2×1). The adsorption characteristics depend strongly on the availability of active sites at the Si(100) surface. Reactivity is enhanced byproductionof active sites during ion prebombardment. Adsorption of C3H6to a disordered, ion‐bombarded Si(100) surface results in nearly complete dissociation of C3H6for ion fluences as small as 1015Ar+ions/cm2. In contrast, for a thermally annealed and ordered Si(100) surface, only 65% of the C3H6dissociates. The remainder of the propylene chemically bonds to the surface as an undissociated molecule which desorbs intact at 550 K. The increase in reactivity is due to an increase in dissociative chemisorption which occurs at defect sites produced by ion bombardment. Reactivity is suppressed bycappingof active sites using atomic hydrogen preadsorption. Hydrogen passivates the Si(100) surface by occupation of silicon dangling bonds, which prevents adsorption of C3H6. By controlling the number and kind of active surface sites in this way, it is possible to manipulate the reactive ability of the Si(100) surface.