The nature of the Co (or Ni) and Mo sulfided species obtained starting from gamma -Al2O3-supported Anderson heteropolyoxomolybdates (CoMo6 and NiMo6) has been analyzed by means of the extended X-ray absorption fine structure (EXAFS) technique with the aid of temperature-programmed reduction (TPR) and spectroscopic measurements. Results are discussed in relation to the adsorption isotherms and to the catalytic activity for the thiophene hydrodesulfurization (HDS) reaction. A comparison with traditional catalysts has been made.