The effects of external electric fields on the properties of halogens, F, Cl, and Br. chemisorbed on Ag(111) surfaces and of CO chemisorbed on Cu(100) and Pd(100) have been investigated theoretically. These studies have been based on the use of ab initio electronic wavefunctions for cluster models of the adsorbate-substrate system. An applied electric field induces a large change in the bond distance above the surface for an ionic adsorbate, eg X/Ag(111), X = F, Cl, or Br. However, it has only a very small effect on the metalCO and CO bond distances for covalently bonded CO/Cu(100) and CO/Pd(100). An applied field also induces large shifts in the halogen-substrate vibrational frequency for X/Ag(111), ω(AgX), and in the CO stretch, ω(CO). Two concepts are developed to explain the shifts in the adsorbate vibrational frequencies. The first of these is a "wall" effect which is shown to apply for both halogens and CO. The wall effect involves the non-bonding penetration of the surface charge by the adsorbate. As this penetration increases, the frequency shifts to larger values and as the penetration decreases the frequency shifts to smaller values. For covalently bonded CO, there is an additional effect due to the π back-donation from the substrate to CO (2π*). The field-induced shifts in ω(AgX) arise from changes in the wall effect because the field moves the halogen toward or away from the surface, depending on the sign of the field. The wall effect is not important for field-induced shifts in ω(CO) because the field does not significantly change the CO distance. Further, the field does not significantly change the substrate π back-donation. The origin of the ω(CO) shifts is the electrostatic interaction of the electric field with the CO dipole moment; this interaction can be treated, to a good approximation, with first-order perturbation theory. In our studies, we have not found any evidence that the field causes changes in the metal-halogen or metalCO chemical bonding. © 1991.