Effects of copper phase on oxidation of carbon monoxide over supported Wacker-type catalysts were examined. XRD patterns and XANES spectra of the catalysts were measured to determine the bulk phase and the chemical environment of copper. The catalytic activity was affected by the composition of copper precursors, the nature of supports, the presence of HCl and SO2 in the feed, and the reaction temperature. All these variables affecting the catalytic activity influenced the XRD intensity of Cu2Cl(OH)(3) phase in the same manner, which was believed to be the active copper phase of the catalyst. Thus the XRD intensity of Cu2Cl(OH)(3) of a catalyst was closely correlated with its catalytic activity and the change in the catalytic activity of CO oxidation with respect to different reaction and preparation conditions was directly related to the behavior of the active copper phase with respect to those variables.