Monometallic An, Pd and bimetallic AuPd nanoparticles were prepared from Au(III), Pd(II) precursor ions in aqueous media with reduction by a mixture of Na-citrate and tannin producing stable metal sols of narrow size distribution. The nanoparticles of 4-7 nm in diameter as indicated by transmission electron microscopy were adsorbed on TiO2. In situ X-ray absorption spectroscopy (XAS) at Au L-III and Pd K-edges and X-ray diffraction (XRD) techniques evidenced the presence of bimetallic particles in the supported AuPd/TiO2 sample. The high amount of organic residues of the samples was removed by calcination at 400 degreesC proved by temperature programmed oxidation (TPO). After reduction in H-2 the TEM, XRD and the CO chemisorption showed some enlargement in the metal particle sizes. The catalytic activity of the bimetallic AuPd/TiO2 in the CO oxidation revealed a slight synergistic effect compared to the activity of monometallic analogous referred to the estimated surface area of Au and Pd in the bimetallic sample. (C) 2003 Elsevier B.V. All rights reserved.