The deactivation of 12.5% Mo/Al2O3 catalysts nitrided at various temperatures during the hydrodesulfurization (HDS) of dibenzothiophene at a 10.1 MPa total pressure was studied on the basis of a simulation using the kinetic equation and the behavior of nitrogen release (N/Mo atomic ratio), sulfidation (S/Mo), and carbon deposition (C/Mo) by X-ray photoelectron spectroscopy (XPS). The nitrided catalysts were deactivated in two regions where the activity significantly decreased up to 3 h and then very slowly in the period of 3-14 h. The catalyst deactivation during dibenzothiophene HDS followed the equation: Phi = r(1) exp(-alpha(1)t) + r(2) exp(-alpha(2)t), where r and alpha are constants, subscripts 1 and 2 denote the rapid and slow deactivation species, respectively, and t is the time-on-stream. The rapid initial deactivation (alpha(1)) was mainly attributed to carbon deposition on the surface, while the slow deactivation (alpha(2)) was significantly associated with nitrogen release. The highly nitrided Mo/Al2O3 catalyst (nitriding at 973 K) exhibited the least deactivation and longest catalyst lifetime. (C) 2003 Elsevier B.V. All rights reserved.