Chemisorption probabilities (S) of monoenergetic I-2, Br-2, Cl-2, and C6H5Cl beams have been measured on the Si(100) (2x1) surface. The sticking probabilities (S) were measured as a function of the incident translational energy (E(i)), the surface temperature (T-s), the angle between the incident beam and the surface normal (theta(i)), and the surface adsorbate coverage (theta). All three diatomic halogens can adsorb via both precursor-mediated and direct-activated chemisorption, while C6H5Cl adsorbs only by precursor-mediated chemisorption. For the diatomic halogens, precursor-mediated chemisorption is most significant for I-2, followed by Br-2, and Cl-2. The barriers to direct-activated chemisorption are seen to increase from similar to 0.055 eV for Cl-2, to similar to 0.12 eV for Br-2, to similar to 0.14 eV for It, For the diatomic halogens, the largest initial sticking probabilities are obtained at the highest incident energies, with S-0 equal to 95-100% for I-2 and Br-2 and 80-85% for Cl-2. The initial sticking probability, So, is independent of the incident angle of the molecular beam for all incident energies for Cl-2 and for C6H5Cl. For all incident kinetic energies, the chemisorption probability decreases linearly with coverage (S = S-0(1 - theta)) for the three diatomic halogen gases. This suggests that a single site is required to initiate the chemisorption process.