In the following we will illustrate these concepts and more recent developments using structurally well-defined PdAg and PtAg surfaces as example, which have been investigated extensively both in our groups as well as in others . These surfaces include Pd and Pt surfaces modified by ultrathin Ag films or surfaces covered by monolayer surface alloys, where a pseudomorphic PdAg or PtAg layer is supported on a Pd or a Pt(111) substrate, respectively. Alternatively, such surface were prepared also by deposition of Pt or Pd on Ag substrates . In an atomic picture, these surface alloy systems differ from the surface region of comparable bulk alloys by the homogeneous local composition and structure of the underlying second and deeper layers, which simplifies the identification and classification of different adsorption sites enormously. Nevertheless, we will compare these results also with comparable data obtained in adsorption studies performed on PdAg bulk alloys , while no such studies were found for PtAg bulk alloys. In the following, we will discuss tests of the adsorption properties of various PtAg and PdAg surfaces using CO and hydrogen as probe molecules / atoms, and compare them with structural information to identify local adsorption properties on these surfaces.