Effects of reaction variables and catalyst components were studied on the activity of supported Wacker-type catalysts for low-temperature CO oxidation, The PdCl2-CuCl2 catalysts supported on alumina or carbon were active in the conversion of CO at ambient conditions and were also stable for at least 100 h as long as water was supplied with the reactant gas mixture. Unlike homogeneous Wacker systems, the active phase of the copper was solid Cu2Cl(OH)(3) as seen by X-ray diffraction, Silica was an inactive support because it was not able to stabilize the active copper phase. Carbon was a more active support than alumina. The activity of alumina-supported PdCl2-CuCl2 showed a marked dependence on the partial pressure of water whereas the carbon-supported catalyst showed no dependence. This difference in catalytic behavior appeared to stem from the catalysts' different interaction with water