The work presented in this paper is a continuation of our earlier work aimed at acquiring a fundamental understanding of sulfided Ni-Mo/gamma-Al2O3 catalysts in HDN catalysis, A series of temperature-programmed desorption and reduction experiments were performed over in-situ sulfided bare gamma-alumina support, alumina-supported monometallic Ni, Mo, and bimetallic NiMo catalysts. The TPD and TPR results were linked to the active site assignment of Ni- and Mo-associated centers in HDN reactions. Postreaction XPS and SEM characterizations were performed for catalysts used in HDN for up to 1000 h. One of the major changes during the aging of the Ni-Mo/gamma-Al2O3 catalysts was the loss of some active sites associated with the Ni promoters, The decreased Ni content over the catalyst surface was consistent with the observed decrease in the hydrogenation function of the catalyst. Two possible mechanisms are suggested for the loss of Ni active sites, One is coking that spreads from the alumina support to the nickel sites. Another is segregation of some Ni atoms from the Ni-Mo-S structure due to loss of structural sulfur,