Dimethyl carbonate (DMC) synthesis by oxidative carbonylation of methanol has been studied using vapor phase flow reaction system in the presence of CuCl2/NaOH/AC (AC, activated carbon) catalysts. A series of CuCl2/NaOH/AC catalysts have been prepared by the conventional impregnation method with varying the molar ratio of OH/Cu from 0.0 to 1.0. The catalytic activity Of CuCl2/NaOH/AC catalysts has been found to increase continuously with the increase of OH/Cu molar ratio as compared with CuCl2/AC catalyst. The morphological analysis by X-ray diffraction, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) has shown that the improved performance of CuCl2/NaOH/AC catalysts was closely related with the existence of copper chloride hydroxides (Cu-2(OH)(3)Cl). Two different crystal habits of Cu-2(OH)(3)Cl were observed in the catalyst surface based on the OH/Cu molar ratio. One is alpha-Cu-2(OH)(3)Cl (atacamite) which is orthorhombic, while the other is gamma-Cu-2(OH)(3)Cl (paratacamite) which is rhombohedral. The catalyst phase was revealed to change from alpha-Cu-2(OH)(3)Cl to gamma-Cu-2(OH)(3)Cl with the increase of OH/Cu molar ratio. The catalyst phase was alpha-Cu-2(OH)(3)Cl dominant at the molar ratio of OH/Cu = 0.5, while at the molar ratio of OH/Cu = 1.0, gamma-Cu-2(OH)(3)Cl dominant. The gamma-type catalyst was found to be more favorable to DMC synthesis than the alpha-type catalyst. The CO temperature-programmed desorption (TPD) experiments on the employed catalysts also strongly supported these observations. (C) 2003 Elsevier B.V. All rights reserved.