<PICT:0769346/III/1> In the catalytic destructive hydrogenation of crude hydrocarbon oil to obtain gas oil, the temperature of the reaction, which is carried out at a pressure of 200 to 750 p.s.i., is controlled by withdrawing oil from the hydrogenation zone, cooling the withdrawn oil, and then returning the cooled oil to the hydrogenation zone. The catalyst may be a metal of Group IV, V, VI or VIII of the periodic system, an oxide or sulphide of these metals, or a mixture thereof; the catalyst may be supported on activated alumina. The catalyst is separated from the products of hydrogenation, and is then regenerated for re-use by burning off the carbonaceous deposits with an oxygen-containing gas, followed if necessary by treatment with chlorine or with aqua regia which has been diluted with water. A sulphurous crude oil introduced through line 1 and hydrogen-containing gas supplied through line 2 are mixed in line 3; and the mixture is passed through furnace F. To the heated mixture is then added, in line 7, tail gas from reactor 8 and recycle oil together with a slurry of catalyst (cobalt molybdate supported on activated alumina) in oil from line 6. The mixture is charged to the bottom of reactor 8 which is maintained at a temperature of 700 DEG -850 DEG F. and a pressure of about 400 p.s.i.g. The expanded upper part of the reactor acts as a chamber which causes the catalyst to settle to the lower part, back-mixing being prevented by a plurality of perforated baffles B. The product is withdrawn from the reactor through line 13 and is charged, via pressure-reducing valve V, into distilling column 14, where it is fractionated under atmospheric pressure, steam being introduced into the column through line 16. Light hydrocarbons and steam are withdrawn from the head of the column, gasoline is withdrawn through line 18, and the main product-gas oil-is withdrawn via line 23. These products have a very low sulphur content. A bottoms fraction containing catalyst is taken off through line 24, and a portion of this fraction is pumped through line 27 to cooler 28 which contains coil 29 through which cold water is circulated. A portion of the material in the reactor is withdrawn from a middle-section via line 31, and this also passes through line 27 to the cooler. The cool material leaving the cooler is pumped to the reactor through lines 10 and 6. A portion of the bottoms fraction in line 24 is passed to settler 33, where the catalyst separates out and clear oil is decanted off through line 34. Since it contains a substantial proportion of the metallic contaminants present in the original feed, it is rejected from the system and may be used as fuel. A small proportion of the slurry withdrawn from the base of the settler is rejected through line 36, but the main portion is either returned to the reactor via line 37 and cooler 28, or is passed through cooler 40 to washing vessel 43 together with naphtha introduced through line 41. The naphtha removes heavy oily material from the catalyst. The solution of this heavy oil is withdrawn through line 48; and the naphtha may be recovered from it by distillation. The cleaned catalyst is withdrawn from the bottom of washer 43 as a slurry, part of which is pumped through lines 47 and 6 to the reactor. The remainder of the slurry is carried, by steam, up line 50 and through a reducing-valve into cyclone S. Dry catalyst collects at the base of the cyclone, and a mixture of steam and oil vapours pass off overhead. The dry catalyst is suspended in air in line 51, and is carried to regenerator 53 where it forms a dense fluidized bed while the carbonaceous and sulphurous deposits are removed therefrom by combustion. Cooling means 54 prevents the regeneration-temperature rising above 1200 DEG F. The catalyst may also be subjected to "rejuvenation," i.e. it is allowed to remain in contact with air at 1050 DEG -1200 DEG F. for 4 to 24 hours. The regenerated catalyst is withdrawn from the regenerator via line 56, passed through cooler 57 to cool it to about 600 DEG -800 DEG F., and mixed with feed-oil from line 58. The slurry, so formed, is pumped to the reactor. When regeneration and rejuvenation fail to restore the activity of the catalyst, a portion thereof is withdrawn from line 56 and is mixed in line 61 with aqua regia which has been diluted with water, the mixture being passed to treating zone 63. The treated catalyst is washed with water in zone 65 in order to remove acid; and the catalyst/water slurry is charged, together with feed-oil introduced through line 67 to column 14. Specification 497,382 is referred to.