Bimetallic Pt-Ag particles dispersed on a carbon powder (Pt-Ag/C) with 1:1, 1:2, 1:3, and 1:10 (atomic ratio) Pt:Ag were investigated as electrocatalysts for the oxygen reduction reaction (ORR) in alkaline media. These materials were characterized by X-ray diffraction, in situ X-ray absorption near-edge structure, and extended X-ray absorption fine structure. The electrochemical activity for the ORR was evaluated from steady-state polarization measurements in a rotating ring-disk system and in gas diffusion electrodes (EDG). The results showed that the Pt-Ag particles are formed by a core composed by true Pt-Ag alloys, with the surface covered by a Ag-rich layer. An increase of the particle size and of the fcc unit cell lattice parameter with the increase of Ag content was observed. The electrochemical measurements indicated lower activity for the Pt-Ag/C particles compared to Pt/C, but higher activity compared to Ag/C. A number of electrons close to 4 was observed for the ORR in the Pt-Ag/C materials. A discussion of the catalytic activity of the Pt-Ag materials was made in terms of the electronic properties of the Pt and Ag atoms and the morphological structure of the particles. (c) 2005 The Electrochemical Society. All rights reserved.