Zeolite-supported cobalt sulfide catalysts, CoS/NaY and CoS/USY-H, were prepared using CO(CO)(3)NO as a precursor. A SiO2-supported cobalt sulfide catalyst was prepared by an impregnation method for comparison. The prepared catalysts were characterized by magnetic properties and XAFS. The cobalt sulfides in CoS/NaY are paramagnetic and can be characterized as intrazeolite dinuclear species with antiferromagnetic interactions between the Co atoms, in contrast to the cobalt sulfides in COS/SiO2, in which 95% Of CO is Co9S8 and 5% is paramagnetic with a very small amount of ferromagnetic species. The structure of the cobalt sulfides in CoS/USY-H depends on the treatment after the sulfidation. Ultra-highly dispersed cobalt sulfide species initially formed during the sulfidation are transformed to highly dispersed Co9S8-like particles and eventually to ion-exchanged Co2+ cations as the evacuation temperature and duration are increased. The catalytic activity of the cobalt sulfide catalysts was measured for the hydrodesulfurization of thiophene. The results suggest a synergy generation between cobalt sulfides and surface hydroxyl groups. (C) 2006 Elsevier B.V. All rights reserved.