The great current interest in hydrodesulfurization (HDS) and other hydrotreating reactions is related to the need for efficient upgrading of crude oil fractions or coal-derived liquids. The catalysts used for such reactions generally consist of molybdenum (or tungsten) supported on high surface area aluminas with cobalt or nickel added as promoters. Great efforts have been devoted to the understanding of the structural and chemical form in which the different elements are present in the active catalyst and to the establishment of correlations between such information and the various catalytic functions. This massive research effort has given valuable information on many aspects of such catalyst systems (for recent reviews of the extensive literature, see, e.g., Refs. 1–11). However, it has not been possible to reach general agreement on the types of structures present in the active catalysts and the origin of promotion