The preparation of hydrotreating catalysts by the impregnation of alumina with solutions of heteropolyanions has been studied. These anions have the general structure [HwAxByOz]n-, where A may be Co or Ni and B may be Mo or W. The anions have advantage over conventional impregnating agents in that both catalytically active metals are in the same complex and should be deposited in the same position in the pellet. In addition, the solubility of the anion is higher than conventional impregnating agents. This allows higher concentrations of catalytic metal (and particularly of tungsten) to be deposited on or in the support. Conditions for the preparation of "shell" or "uniform" impregnated catalysts have been established. Comparisons have been made of the efficiency of such catalysts and of conventional catalysts for the hydrotreating of a model feed-stock. Heteropolyanion-based catalysts are very effective for hydrodesulphurization and hydrogenation but their relatively low content of promoter metals restricts hydrodenitrogenation activity.