Hydrocarbon materials such as mineral oils and tars from coal, shale, lignite, or peat are freed from phenols and like oxy-bodies by heating under pressure in a closed vessel to a temperature and for a time sufficient to effect reduction of the oxy-bodies, and then removing the water formed by the reaction. 350/sup 0/ to 400/sup 0/C. for 30 to 60 minutes is suitable. Any wax-like constituents are converted to liquids of lower viscosity and settling point. The product may be fractionated to give light oils and a residue of aviation Diesel fuel. In an example, oil from the low-temperature distillation of coal and having a tar acid content of 30 per cent is treated in a tubular converter at 380/sup 0/C. and 400 lb. per sq. in for 40 min., and the benzine toluol, and xylol distilled; the residue has a tar acid content of only 7.6 per cent.