We investigated an application of Xe-129 NMR to the analysis of sulfided Co-Mo/Al2O3 catalysts. For a mixture of Mo/Al2O3 and Co/ Al2O3, two signals appeared in the Xe-129 NMR spectrum, while only one signal appeared in the spectrum of other catalysts such as Mo/ Al2O3 and Co-Mo/Al2O3. The downfield signal was attributed to Mo/Al2O3 and the other one was attributed to Co/Al2O3. This result demonstrates that xenon atoms adsorbed on Mo/Al2O3 make few exchanges with ones adsorbed on Co/Al2O3 in the mixture during the NMR measurement. For sulfided Co-Mo/Al2O3, the chemical shift delta of the signal varied nonlinearly against the amount of adsorbed xenon, N. This suggests that the electrons of xenon strongly interacts with coordinative unsaturated sites at the edges of the stacked MoS2 crystallite. Furthermore, we calculated the term delta(0) that is mainly dependent on collisions between xenon and other atoms by fitting the delta curve against N to the theoretical equation. delta(0) became gradually larger with the increasing Co/Mo ratio and reached its maximum value at Co/Mo = 0.71. This behavior is mainly caused by a growth of the term delta(M) that is dependent on the magnetic susceptibility. Such results suggest the formation of the Co-Mo-S phase, indicating the antiferromagnetism. On the other hand, So slightly decreased at Co/Mo = 0.99. This is closely related with the formation of not only the Co-Mo-S phase but also Co9S8, indicating the diamagnetism. Hence, delta(0) is a parameter that represents the formation of the Co-Mo-S phase, and Xe-129 NMR can be a powerful tool for analysis of the Co-Mo-S phase on Co-Mo/Al2O3 hydrodesulfurization catalysts. (c) 2005 Elsevier B.V. All rights reserved.