The understanding of the surface properties versus bulk properties of supported metal and alloy nanoparticle catalysts is essential for exploiting their unique properties for applications in hydrogen storage, fuel cells and sensors. This poster shows the recent findings of an investigation of CO adsorption and oxidation on SiO2-supported Au, Pt and AuPt nanoparticles with controlled sizes (2-5 nm) and alloy compositions, which were prepared by molecular encapsulation and thermal treatment. CO adsorption was characterized using an FTIR technique, whereas CO oxidation was monitored using a fixed bed setup with gas chromatographic detection. The surface and bulk properties of the nanoparticles were also studied using XRD, TEM, and electrochemical techniques. The results revealed some initial insights into the understanding of the relationship between the catalytic activity and the surface characteristics of the metal and alloy nanoparticles. Implications of the findings to the design of nanostructured catalysts are also discussed.