During the past decade, alloy surfaces have been increasingly studied on the atomic level. As a result, their atomic structure, composition and the adsorbate-surface interactions are much better understood. The purpose of this paper is to review the status of our present knowledge of the chemical properties of single crystal surfaces of Pd and Pt alloys. In particular, the adsorption of H 2, 0 2, CO, NO and the catalytic reactions involving these molecules on single crystal surfaces of Pd-Cu , Pd-Ag , Pd-Au , Pt-Cu , Pt-Rh , Pt-Fe , Pt-Co , Pt-Ni and Pt-Ti are discussed and compared with the processes on the corresponding single crystal surfaces of the pure metals.