Results on the catalytic oxidation of CO at 523 K over bimetallic Pd-Ag catalysts supported on pumice are reported and compared with those obtained from the monometallic Pd and Ag catalysts. The catalysts were prepared by organometallic precursors and were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) after different treatments with air and with hydrogen. The activity of the catalysts decreases with increasing Ag/Pd atomic ratio,. the monometallic palladium exhibiting the highest activity. Furthermore, the treatment with air and with hydrogen always results in a better catalytic performance. On the basis of the structural characterization by XRD and the surface atomic composition obtained from XPS measurements, Ag has tendency to segregate at the surface of the catalysts. except in air environment at 623 K. In this case, an inward diffusion of silver atoms occurs. By hydrogen treatment at high temperature. silver reappears at the surface with further increase of the catalytic activity. The results are explained in terms of rearrangement of the bimetallic particles with the most active situation represented by clean particles of pure palladium next to dispersed silver particles.