The chemical states of the active species, Co and Mo, of the Co-Mo/B2O3/Al2O3 catalysts were investigated during the sulfidation treatment by means of an in situ X-ray absorption fine structure technique. It was clearly shown by Co K-edge X-ray absorption near edge structure (XANES) spectra that the sulfidation of Co is strongly suppressed by the addition of citric acid (CA) to the catalyst. Mo K-edge XANES spectra revealed that the sulfidation of Mo is also inhibited by the addition of CA. The sulfidation behavior of Mo depended on the catalyst preparation method. With the Co-Mo catalyst prepared by the post-treatment of a calcined Co-Mo catalyst with CA, Mo was sulfided prior to or in parallel with the sulfidation of Co, resulting in a higher activity of the catalyst for the hydrodesulfurization of thiophene than that of a catalyst prepared by simultaneous impregnation of Co, Mo, and CA. (C) 2009 Elsevier B.V. All rights reserved.