A heterogenized Wacker catalyst system in which pores of a high surface area alumina were filled with an aqueous solution of PdCl2CuCl2 was active for the oxidation of CO near room temperature. The structure of the catalyst was studied by X-ray absorption fine structure (XAFS). The active phase of Pd was a Pd11 species containing chlorine and, probably, carbonyl ligands. Direct interaction of PdPd or PdCu was not detected. The active phase of copper was found to be solid Cu2Cl(OH)3 particles in agreement with the X-ray diffraction (XRD) results. The presence of Cu was essential to keep the Pd in the Pd11 state during the reaction. The rates of CO oxidation measured at temperatures of 30–70°C showed a minimum at 40°C, which was attributable partly to an unusual structure change of the active palladium species during the reaction at this temperature.