Oil shale from the Kimmeridge Clay, of Jurassic age from the UK was pyrolysed in a 5 kg fixed bed reactor at 525 degrees C in a nitrogen atmosphere. The derived shale oil was then hydrotreated at 15.0 Mpa pressure and 400 degrees C in a stirred reactor with a nickel-molybdenum (Ni-Mo) catalyst and residence times from 8 to 56 h. The shale oils were analysed for polycyclic aromatic hydrocarbons (PAH) and for nitrogen-PAH (PANH) and sulphur-PAM (PASH), before and after hydrotreatment. The results showed that generally the higher molecular weight three and four ring PAH decreased with increasing hydrotreatment time, however, single ring aromatic compounds and two ring PAM were increased. Nitrogen and sulphur containing PAH were significantly reduced in concentration in the oils with increasing hydrotreatment time to reach negligible concentrations after 56 h. The reduction in PANH and PASH coincided with a reduction in the overall nitrogen and sulphur contents of the oils. O 1999 Elsevier Science Ltd. All rights reserved.