Hydrofining and hydrotreating are processes in which selected petroleum fractions react with hydrogen in the presence of monofunctional catalysts: metallic sulfides or oxides. The purpose of hydrofining is the elimination of the heteroatoms by means of hydrogenolysis reactions. In hydrotreating, which uses more severe operating con­ ditions and more active catalysts, the polycylic aromatic hydrocarbons also undergo a partial hydrogenation.