The deactivation behavior of Co(Ni)MoP/Al2O3 catalysts during diesel fuel hydrotreatment is investigated by means of X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). XPS measurements performed on aged Co(Ni)MoP/Al2O3 catalysts show that the promoter quantity on the edges Of MoS2 nano-crystallites decreases with increasing lifetime. TEM coupled with energy dispersive X-ray spectrometry (EDX) indicates that the Co(Ni)/Mo ratio inside crystallites changes without any modification over MoS2 layers (size and stacking). Moreover toluene hydrogenation activity loss on aged catalysts is found to be linearly correlated to the XPS promotion rate decrease during deactivation. These experimental results have been analyzed in the light of recent density functional theory (DFT) calculations on the thermodynamic stability of Co(Ni)MoS phases in sulfo-reductive working conditions. It is shown that the destabilization of the mixed phase may be partially attributed to the reaction conditions (high temperature and reductive environment). In addition, the role of the coke cannot be excluded to explain the promotion rate decrease revealed by XPS spectra. The experimental results combined with DFT results are used to propose an origin of the active phase modifications in working state. (C) 2007 Elsevier B.V. All rights reserved.