Upgrading of Estonian shale oil fraction boiling at 240-320 degrees C, the so-called light mazute characterized by sulfur content 0.62% and polar heteroatomic compound content 30.8% after dephenolation, was effected by hydrogenation in an autoclave at 370 degrees C using a Co-Mo catalyst. The composition and properties of initial, dephenolated and hydrogenated light mazute were determined. Olefinic double bonds in oil were totally saturated and most oxygen and sulfur were eliminated as a result of hydrogenation, 90% of oil consisting in hydrocarbons only.