Diethyl carbonate (DEC) is a candidate for use as an oxygen-containing additive in gasoline and diesel fuel to diminish pollutant emissions. The synthesis of DEC by the oxidative carbonylation of ethanol in the gas phase over heterogeneous CuCl2PdCl2 catalysts supported on activated carbon (AC) has been investigated using a laboratory-scale continuous flow reactor with online GUMS. Influences of various reaction conditions and catalyst pretreatment on the DEC yield and selectivity have been tested. Yield of DEC at 150 degreesC reached a maximum of 12.5 wt.% and was found to increase with an increase of residence time, reaction temperature, and reaction pressure as expected. The relationship between CO flow rate and production of DEC showed three distinct regions of DEC production. A by-product, diethoxymethane (DEM), was formed when ethanol was introduced in stoichiometric excess. Catalyst pretreated with KOH presented the best catalytic performance, and all metal hydroxides tested enhanced the yield of and selectivity for DEC simultaneously. However, the CuCl2/PdCl2/AC-KOH catalyst with a higher OH/Cu mole ratio than 2.0 showed an even lower DEC yield than that found using a CuCl2/PdCl2/AC catalyst without KOH. (C) 2003 Elsevier Science B.V. All rights reserved.