UK oil shale was pyrolysed in a fixed bed reactor at 525 degreesC to produce shale oil. The pyrolysis oil contained high concentrations of nitrogen and sulphur compounds. The shale oil was subsequently catalytically hydrotreated to remove the nitrogen and sulphur compounds to produce an upgraded oil. Catalytic hydrotreatment was carried out at 15.0 MPa pressure and 400 degreesC with a Co-Mo catalyst. The influence of hydrotreatment reaction time on the concentration of nitrogen and sulphur compounds in the hydrotreated oils was investigated. Aromatic nitrogen and sulphur compounds were found to be significantly lowered after catalytic hydrotreatment and were further lowered as the hydrotreatment reaction time was increased. The reduction in aromatic nitrogen and sulphur compounds coincided with a reduction in the overall nitrogen and sulphur contents of the oils. Hydrogenation of the oils was apparent through a decrease in the alkene content and an increase in the hydrogen content of the hydrotreated oils. However, the content of aromatic hydrocarbons in the oils increased after hydrotreatment and was further increased with increasing reaction time. The more polar fractions of the untreated oil were found to decrease after hydrotreatment suggesting that hydrogenation of these polar fractions resulted in an increased aromatic content of the oils.