Nitric oxide reduction by hydrogen has been studied on Pt-Ru/SiO2 catalysts of various Pt/Ru atomic compositions in the temperature range 298-673 K. Physical characterization showed the presence of bimetallic particles which tend to be Pt-rich. The overall activity of the bimetallic catalysts suggests a dilution of the active component (Pt) in the range 373-523 K. The addition of Ru results in a general improvement of the N-2 selectivity and significant modifications in the product distribution are observed as a function of the catalyst composition. A bimetallic particle model is proposed in which various types of surfaces are exposed including those with pure Pt atoms and/or Pt-Ru mixture. This model allows to explain the overall activity and selectivity in the whole series of catalysts.