A heterogenized Wacker catalyst system in which pores of a high surface area alumina were filled with an aqueous solution of PdCl2-CuCl2 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 Pd-II species containing chlorine and, probably, carbonyl ligands. Direct interaction of Pd-Pd or Pd-Cu 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 Pd-II state during the reaction. The rates of CO oxidation measured at temperatures of 30-70 degrees C showed a minimum at 40 degrees C, which was attributable partly to an unusual structure change of the active palladium species during the reaction at this temperature.