When prepared properly, particles of Au, about 2-3 nm in size, are extremely active for low temperature CO oxidation. When free of impurities and in a wet atmosphere, the activity depends relatively mildly on the nature of the support. Yet the dependence of the support increases markedly in a dry atmosphere. Understanding these phenomena and what makes Au catalysts active could lead to development of new and much more active catalysts for destruction of pollutants. We have been investigating the nature of the active sites on Au catalysts and the reaction mechanism using a combination of in-situ spectroscopic techniques, such as FTIR, X-ray absorption (EXAFS and XANES), poisoning, and transient measurements.