An X-ray diffraction study was carried out on powders of a series of catalysts prepared from aqueous solutions of H2PtCl6 and PdCl4 and amorphous SiO2 with different concentrations in weight of Pt and Pd at about 4% in overall metallic weight. Measurements of the position of high angle Bragg reflections in the diffractograms show evidence of the fact that the small catalyst particles are bi-metallic Pt-Pd crystals having a face-centred cubic Bravais lattice. The lattice constant of these crystals was found to change with the relative concentration of Pt and Pd by following the Vegard's rule. This correlation leads to the conclusion that the bi-metallic catalyst particles are made of a single solid solution of Pt and Pd atoms in the whole range of relative concentrations. Relative concentrations of these metals in the samples under study were determined by using energy dispersive X-ray spectrometry and their values were found to be close to the stochiometric relative concentrations in weight of the metals in the precursor aqueous solution. An average size of about 96 Angstrom was estimated for the bi-metallic particles from the full-width at half-maximum value measured for the (2 0 0) diffractometric curve. (C) 2003 Elsevier B.V. All rights reserved.