The auto-reduction of copper and manganese acetates has been investigated using in situ X-ray diffraction and thermogravimetric analysis, with the intention of manipulating the phenomena to tailor specific phase formation for synthesising catalysts. Subsequently catalysts prepared in this controlled manner were evaluated for ambient temperature CO oxidation. The decomposition of mixed copper and manganese acetate systems was controlled to form MnO(x)-supported Cu or CuMnO(x), spinet structures, depending on the oxygen concentration and flow conditions during the heat treatment. Catalyst precursors were prepared by physical grinding and by a supercritical CO(2) anti-solvent precipitation process. The use of supercritical anti-solvent precipitation allows for the formation of well-mixed metal acetates that decompose to form active spinel CO-oxidation catalysts or small copper nano-particles supported on MnO(x), depending on the oxygen content of the heat treatment atmosphere. The ability to tune oxidation state and phase composition of catalysts is a key preparation parameter for controlling the activity and provides insight into the active sites for CO oxidation. (C) 2011 Elsevier Inc. All rights reserved.