Co−Mo/B2O3/Al2O3 catalysts were prepared by a post-treatment of the calcined states with citric acid (CA) and were tested for the hydrodesulfurization (HDS) reaction of thiophene. The catalysts were characterized by LRS, UV−visible spectroscopy, Mo K-edge EXAFS, TEM, NO adsorption capacity, and surface area measurements. Catalysts obtained by simultaneous impregnation with citric acid were also prepared for comparison. Sulfided catalysts were characterized by a chemical vapor deposition (CVD) technique using Co(CO)3NO. The addition of citric acid in a post-treatment more effectively increased the catalytic activity and NO adsorption capacity than did addition by a simultaneous-impregnation method. Co-CA and Mo-CA surface complexes formed at the expense of CoMoO4 and polymolybdates. The maximum potential activity of Co−Mo/B2O3/Al2O3 was obtained by a simple post-treatment of the calcined catalyst with citric acid.