The kinetics of the oxygen reduction reaction (ORR) was studied on ultra thin layer electrodes formed by bimetallic Ag-Co nanoparticles dispersed on a carbon powder (Ag-Co/C) in KOH electrolyte. The morphological features of the materials were studied by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Cyclic voltammetry and steady state polarization measurements for the ORR were obtained using the rotating ring/disk electrode (RRDE) technique. TEM measurements for the bimetallic Ag-Co/C catalysts showed a heterogeneous crystallite size distribution and, together with XRD, suggested the presence of a segregated phase of cobalt, mainly as Co3O4. The characterization of the materials by XRD also showed a negligible decrease in the Ag face centered cubic (fcc) lattice parameter, which was ascribed to a negligible Co insertion into the Ag structure. Levich plots obtained from rotating disk electrode data have shown a higher number of electrons for the Ag-Co/C materials, compared to Ag/C. The polarization results showed higher activity for the ORR on Ag-Co/C materials in comparison to Ag/C. (c) 2006 Elsevier B.V. All rights reserved.