We have studied the dependence of the saturation coverage of fluorine on the Si(100)-2x1 and Si(111)-7x7 surfaces upon F-2 incident energy by measuring the desorption yields of SiF2 and SiF4 as a function of F-2 exposure generated by normally incident, nearly monoenergetic molecular beams. When the F-2 molecules have incident translational energy E(i) = 0.060 eV, the desorption yields rapidly approach limiting values with increasing exposure. However, when E(i) 0.275 eV, the SiF4 desorption yields on both surfaces slowly approach limiting values which are increased by nearly a factor of 4 over that for E(i) = 0.060 eV. These observations indicate that although there is no barrier to the dissociative adsorption of F-2 onto the Si dangling bonds of the clean surfaces,’ there exists a barrier, between 0.060 and 0.275 eV, to the insertion of fluorine into the most vulnerable Si-Si backbonds on the saturated silicon surfaces.