The understanding of the surface characteristics of alloy nanoparticles is essential for exploiting their unique catalytic properties. This presentation reports findings of an FTIR investigation of CO adsorption on the surface of silica-supported bimetallic nanoparticles. The CO stretching bands on the silica-supported alloy nanoparticles (2-5nm) with controlled bimetallic compositions (Pt, Au, Fe, Cu, Co, Fe2O3, etc) treated under different temperatures were compared with those of the monometallic counterparts, focusing on the effect of the bimetallic composition on CO stretching bands. For AuPt nanoparticles, the disappearance of the Au-atop CO band and the shift in the Pt-atop CO band for Pt atoms surrounded by Au atoms with increasing Pt concentration in the bimetallic nanoparticles are shown to correlate well with the electronic effect. Implications of the findings to the understanding of the synergistic properties of the gold-based and other bimetallic (e.g., PtFe, PtCo, etc.) nanoparticle catalysts will also be discussed.