The effect of calcination temperature on the electronic structure of gold catalyst was investigated in this paper. CO oxidation was employed as a test reaction to correlate the phase changes to the catalytic activity. The TPR, XPS and XRD characterization showed that the highly active Au/FeOx/Al2O3 catalyst contained both gold atoms and ions. The catalytic performance of gold catalyst may depend on the Au-0/Au3+ ratio. The best activity in the present work was obtained when the Au-0/Au+ ratio was about 0.89. (c) 2006 Society of Chemical Industry