Supported nanoparticle catalysts have been key players in technologies tied to energy production, utilization, and emissions abatement. Classic examples of include platinum metal reforming catalysts, fuel cell catalysts, and the automotive catalytic converter. Although supported nanoparticle catalysts are readily prepared by standard impregnation techniques, active sites are not necessarily well characterized or well understood. Our work is directed towards developing a characterization and better understanding of the active sites in CO oxidation. In this presentation, we discuss characterization of supported Au catalysts prepared using PAMAM dendrimers as nanoparticle templates. The finely dispersed nanoparticles, deposited onto a commercial titania and activated to yield highly active heterogeneous catalysts. The catalysts were evaluated for low temperature (263 293 K) CO oxidation activity, which is an important process for cleaning hydrogen fuel streams for fuel cell applications. Analysis of oxygen dependence and competition experiments at low conversion (< 2%) suggested two distinct CO oxidation reaction mechanisms that yield similar overall reaction rates. "As prepared" commercial catalysts showed a nearly 2nd order oxygen dependence and saturation behavior, while highly reduced samples were approximately 0.2 order in oxygen and did not saturate at 25% oxygen. Mechanistic implications of the apparently different mechanisms will be discussed.