A study of the activity of bimetallic Pd-Rh catalysts supported on silica in the reduction of NO by CO is presented. The catalysts were prepared by three different methods : (1) Pd and Rh were coimpregnated on the support, (2) Rh was impregnated first and, after calcining, the sample was impregnated with Pd, (3) the monometallic Pd and Rh catalysts were physically mixed. The results showed that the activity of the catalysts prepared by coimpregnation was much lower than that of the other two catalysts.