The nature of chemisorbed CF3I and C2Cl4 on silicon surfaces has been studied using a variety of ultra-high vacuum surface-science techniques, including temperature-programmed desorption and electron-stimulated desorption.  The sticking coefficients for both CF3I and C2Cl4 on 370 K silicon surfaces are near unity.  For the CF3I-Si system, SiFx species and I desorb in temperature-programmed desorption.  No molecular CF3I desorbs.  For the C2Cl4-Si system, SiCl2 desorbs in temperature-programmed desorption.  No molecular C2Cl4 desorbs.  Low-energy (< 100 eV) electron-stimulated desorption of F+, F-, Cl+, and Cl- are detected, while only a trace of I+ and no I- are detected.  The low iodine ion yield could be due to the large mass and radius of iodine.  In temperature-programmed electron-stimulated desorption, the ion signals are recorded as the CF3I-Si and C2Cl4-Si surfaces are heated.  This measurement demonstrates the sensitivity of the electron-stimulated desorption towards the state of the surface.  There is evidence of quenching of electronically-excited states by adsorbed iodine.