The adsorption of Cl-, Br- and I- anions on cluster models of the Ag(100) and the Pt(100) surfaces has been studied by means of ab initio wavefunctions. Potential energy curves have been determined for various adsorption sites, allowing a direct comparison of the adsorption properties of silver and platinum surfaces. The bonding mechanism has been analysed in detail by means of a technique which allows one to decompose the interaction energy into the sum of various contributions. Three dominant mechanisms contribute to determine the adsorption energy : the Pauli or exchange repulsion, the metal polarization, and the partial dative bonding from the anion to the surface. On Ag clusters the trend in stability of the various adsorption sites, bridge > four-fold hollow > top, closely follows the order of polarizability of the sites. On Pt, the preferred site is determined by the Pauli repulsion more than by the substrate polarization, providing an indication of the importance of relativistic effects in Pt.