Effect of different preparation methods of MoO3/Al2O3 catalysts on the existing states of Mo species and hydrodesulfurization activity was investigated. A series of supported Mo catalysts were prepared by equilibrium adsorption method (at 298 K or at 373 K) and incipient wetness impregnation method, with ammonium heptamolybdate as Mo precursor, gamma-alumina and pseudo-boehmite as initial support. The results of pre-hydrothermal treatment and XRD characterization showed that different preparation methods can greatly influence the dispersion of Mo species. The results of LRS, TPR, XPS and HRTEM characterization clearly indicated that the existence of Al(OH)(6) Mo6O183- anions can hinder the reduction or sulfidation of Mo species. The HDS tests showed that the catalyst prepared by incipient wetness impregnation method exhibits the highest HDS activity for 4,6-DMDBT, owning to the predominance of well-dispersed polymolybdate species and the absence of Al(OH)(6) Mo6O183- anions. (C) 2013 Elsevier Ltd. All rights reserved.