The colour stability of colour unstable petroleum hydrocarbon fractions boiling in the range 150 DEG -300 DEG C. is improved by contact with a hydrogenation catalyst in the presence of hydrogen at 670 DEG -750 DEG F., preferably 680 DEG -730 DEG F., removing any hydrogen sulphide formed, and redistilling the product to remove not more than 10%, preferably 0,5-5% by weight of bottoms. Preferred conditions are pressures of 50, and especially 500, to 1,000 p.s.i.g., a space velocity of 0,1-20 v./v./hr., and a hydrogen rate of up to 2,000 SCF/B from a lower limit of at least 10 SCF/B. The catalyst suitably consists of 1-8% by weight cobalt oxide, and 5-40% molybdenum oxide supported on alumina which comprises the balance, while the feedstock preferably comprises a straight run fraction or an extract thereof.
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