The effect of citric acid (CA) addition was studied on the HDS of thiophene over Co-Mo/(B)/Al2O3 catalysts. The catalysts were characterized by means of LRS, Mo K-edge EXAFS, NO adsorption capacity measurements, and UV-vis spectra. The catalysts were subjected to a chemical vapor deposition (CVD) technique using Co(CO)(3)NO as a precursor of Co in order to get deeper insights into the effect of citric acid addition. It was shown that the HDS activity was enhanced by the citric acid addition up to the CA/Mo mole ratio of around 1 and leveled off with further addition. The amount of Co anchored by the CVD was increased by the addition of citric acid, suggesting an increase in the dispersion of MoS2 particles on the catalyst by the simultaneous presence of Co, Mo and citric acid, in conformity with the increase in the NO adsorption capacity. In contrast to Co-Mo catalysts, the edge dispersion of MoS2 particles in Mo/B/Al2O3 was not affected by the addition of citric acid. The LRS, UV-vis spectra and Mo K-edge EXAFS showed that Co-CA and Mo-CA surface complexes are formed by the addition of citric acid. The Co-CA Surface complex is more preferentially formed on CoMo/Al than on CoMo/B/Al, in agreement with a greater promoting effect of citric acid at a lower CA/Mo mole ratio for CoMo/Al than for CoMo/B/Al. (c) 2009 Elsevier B.V. All rights reserved.