Evidence is presented and discussed for the involvement of ionic gold, probably Au(1+) in catalysis by supported gold. Various catalytic systems and various experimental techniques have been used for reactions involving carbon monoxide. Mössbauer effect spectroscopy on gold-titania and gold-titania-zirconia reveals the co-existence of Au(0), Au(I) and Au(III), but only the Au(I) content correlates directly with the activity of these solids for CO oxidation. XPS data suggest that a small (ca. 6%) fraction of the gold present in gold-titania-zirconia catalysts is present in an electron deficient state relative to Au(0). For gold–iron oxide catalysts, high water-gas shift activity is obtained for samples in which a partial reduction of the gold, initially present as Au(III), has occurred. For gold-HY zeolites, in which the gold is initially introduced as Au(III) by ion-exchange from [Au(en₂)]³⁺, samples become catalytically active only after a considerable induction period has been exceeded. The induction period is shortened and the activity of the catalysts increased by pre-treatment with sodium borohydride, and some degree of reduction of the gold is assessed to have occurred. Further evidence that non-equivalent states of gold exist (i.e. metallic and ionic gold) in active CO oxidation catalysts based on gold-titania is provided by the fact that partial removal of gold occurs when the solids are treated with solutions containing cyanide ions under oxidative conditions. The specific activity per unit of gold increases with decreasing gold content when the gold content is lowered by such treatment. The results are consistent with the requirement for Au(I) involvement in the catalytic reaction, though a role for Au(0) is not ruled out.