Based on the structure characterization of Au/Fe2O3 catalysts, the mechanism of gold activation in supported gold catalysts was proposed as follow: Au3+ d(8) 6s(0) (catalyst precursor) (activation treatment)-->Audelta+ d(10-delta)6s(1) (catalyst) The possible catalytic active state is the partially oxidized gold (Audelta+) with unoccupied outer d orbitals, similar to the outer d orbital structure of Pt. Thus "inert" gold can become very active for CO oxidation.