Recent intense interest in catalysis by supported gold has focused mainly on the oxidation of CO at low temperatures, which offers the potential for many important applications, including the purification of hydrogen for automotive fuel cells. Numerous, contradictory proposals have been made concerning the nature of the active sites in supported gold catalysts. We now present evidence from a set of complementary experimental methods characterizing Au/alpha-Fe2O3 catalysts, demonstrating that cationic gold plays a crucial role in catalyzing CO oxidation at 298 K, as well as in the hydrogenation of crotonaldehyde. A series of catalysts based on a co-precipitated 5-wt% Au/Fe2O3 precursor heat treated in different ways were compared for CO oxidation at ambient temperature. The catalysts were structurally and chemically analyzed by HREM and STEM-XEDS. A combination of EXAFS, in situ XANES, XPS, and Mossbauer effect spectroscopy demonstrates the important role of cationic gold in the activity of these iron oxide-supported gold catalysts. (c) 2006 Elsevier Inc. All rights reserved.