Pd(II) chloride and Cu(II) chloride in various liquid polymer media supported on silica gel were prepared and used in a catalyst system for vapor-phase synthesis of acetaldehyde by Wacker-type oxidation of ethylene. This catalyst system supported on silica gel prepared by impregnation was quickly deactivated, while use of polyethylene glycol (PEG) as a liquid polymer medium supported on silica gel showed stable catalytic activity. PEG inhibited the formation of Pd metal particles, which deactivate the catalyst system. Addition of alkali metal salts, especially LiCl, to the PdCl2-CuCl2 catalyst system with PEG enhanced catalytic activity for 22 h, even when the I'd content was high, leading to high activity but poor stability. LiCl also inhibited the formation of metal particles. (C) 2008 Elsevier Inc. All rights reserved.