Reactive adsorption desulfurization (RADS) of a model oil n-nonane containing dibenzothiophene (DBT) was conducted over a Ni/ZnO adsorbent; the evolution of Ni/ZnO adsorbent structure during desulfurization was monitored by in situ X-ray absorption spectroscopy (XAS). The reaction mechanism under different atmospheres and the role of hydrogen in the desulfurization were investigated. The results indicated that the desulfurization processes under nitrogen and hydrogen are different in the reaction mechanism. In nitrogen, the desulfurization over Ni/ZnO is achieved through physical and chemical adsorption; a severe decrease in the desulfurization activity of Ni/ZnO is observed with the time on stream and the desulfurization capacity is very low. In hydrogen, the desulfurization turns to be a reactive adsorption process and Ni/ZnO exhibits a high desulfurization activity and capacity. Hydrogen plays an important role in the RADS; it facilitates the decomposition of DBT on active Ni species, the formation of Ni3S2, and thereafter the transfer of sulfur to ZnO. Metallic Ni as the active nickel species is preserved until most of ZnO is converted to ZnS. On the basis of these observations, a possible sulfur transfer mechanism for the RADS is then proposed. (C) 2011 Elsevier B.V. All rights reserved.