The synthesis of diethyl oxalate from ethyl nitrite and carbon monoxide was studied in a continuos flow micro fixed-bed reactor at atmospheric pressure. Two palladium catalysts supported on alpha -Al2O3 of different pore structures were studied under mild conditions. One catalyst (1 wt.% Pd/alpha -Al2O3-1) showed higher catalytic activity and selectivity than the other catalyst (1 wt.% Pd/alpha -Al2O3-2). X-ray diffraction patterns have confirmed that both supports are well-defined alpha -Al2O3. Palladium dispersions were greater on the 1 wt.% Pd/alpha -Al2O3-1 catalyst than that on the 1 wt.% Pd/alpha -Al2O3-2 catalyst as determined by hydrogen chemisorption. The FTIR spectroscopy study indicated that the 1 wt.% Pd/alpha -Al2O3-1 catalyst adsorbed CO easily in the Linear form (band at 2089 cm(-1)) and bridge form (bands at 1950 and 1880 cm(-1)), and as carbonate species (at 1628 cm(-1)). There was little CO adsorption on 1 wt.% Pd/alpha -Al2O3-2. The average pore size of the support alpha -Al2O3-1 was 41.6 Angstrom, and 14.0 Angstrom for alpha -Al2O3-2 This study reveals that the pore structure of the supports remarkably affects the palladium dispersion of the catalysts and alters the CO adsorption behavior, therefore, dramatically affect the catalytic performance. In the case of CO + C2H5ONO reactions, diethyl oxalate was efficiently formed over the 1 wt.% Pd/alpha -Al2O3-1 catalyst.