The high anisotropic character and inherent disorder in the structure Of Supported MoS2-based catalysts that are used extensively to perform hydrotreating reactions for the removal of heteroatoms (S, N, and 0), aromatics, and metals make characterization of the active catalyst a difficult challenge. XAS (X-ray absorption spectroscopy), XRD (X-ray diffraction), and HRTEM (high-resolution transmission electron microscopy) have been widely used in an attempt to understand the structure and origin of the active phase in these catalysts. However, phase and the associated Co promoter when used all these techniques have limitations in determining the structure of the active MoS-individually. Current techniques are notable to provide information of both lateral dimensions along the basal direction and of stacking height Of MoS2 slabs without ambiguity. We report here the use of a synchrotron source for X-ray-scattering measurements of supported MoS2) and C cobalt-promoted MoS2 catalysts. This synchrotron source strongly increases the signal to noise ratio resulting in the detection of diffraction features providing information on the dispersion of the active phase. Synchrotron X-ray measurements in combination with HRTEM can then give a more complete picture of catalyst structure and of the active phases present. Furthermore, supported industrial catalysts that have operated under refinery conditions for more than four years have been studied to better understand the stabilized catalytic phase under these conditions. Industrial hydrotreating conditions induce a "destacking" process resulting in the stabilization of single-layered MoS2-like nanoparticles. This effect has been confirmed On a freshly sulfided model CoMo/Al2O3 catalyst that underwent substantial morphological change leading to the formation of single slabs under HDS conditions. Other structural effects are also reported. This study emphasizes the importance of determining the catalytically stabilized phases under operating hydrotreating conditions as a basis for understanding the activity and selectivity of this class of catalysts. (C) 2004 Elsevier Inc. All rights reserved.