Process for the desulfurization of hydrocarbon oils with water vapor addition to the reaction zone

A continuous process for the catalytic hydrodesulfurization of vanadium-containing heavy hydrocarbon oils which comprises (a) contacting in a reaction zone vanadium-containing heavy hydrocarbon oil at elevated temperature and pressure with hydrogen and with a catalyst which contains from about 0.5 to 20 parts by weight of nickel and/or cobalt and from about 2.5 to 60 parts by weight of molybdenum and/or tungsten per 100 parts by weight of a porous carrier selected from the group consisting of alumina, silica, magnesia, zirconia and mixtures thereof and in the absence of added water vapor until the average vanadium content of the catalyst has increased during said contacting by at least 5 parts by weight per 100 parts by weight of said catalyst and (b) adding and maintaining a quantity of water vapor to the reaction zone after said vanadium content of said catalyst has increased during said contacting by at least 5 parts by weight per 100 parts by weight of said catalyst, said quantity of water corresponding to a water vapor partial pressure in the range from 0.5 to 30 bar during the process.