Oil shales were pyrolysed in a fixed bed reactor and the chemical composition of the oils and gases from the pyrolysis of the oil shales were characterised. The reactor system was then modified to incorporate a second reactor where the derived vapours from oil shale pyrolysis were upgraded on-line using zeolite ZSM-5 catalyst at low pressure. The influence of catalyst temperature from 400 to 550°C, on the yield and composition of the derived oils and gases was determined. In particular, the aromatic and polycyclic aromatic composition of the oils was determined. Gases were analysed before and after catalysis by off-line packed column gas chromatography. The oils were analysed by liquid chromatography fractionation followed by gas chromatography/mass spectrometry. The results showed that the main gases from the pyrolysis of oil shales were, CO 2, CO, H 2, CH 4, C 2H 4, C 2H 6, C 3H 6, C 3H 8 and minor concentrations of other hydrocarbon gases. After catalysis the concentration of all the hydrocarbon gases were increased. The yield of oil after catalysis was reduced with a consequent higher yield of gases and formation of coke on the catalyst. The influence of catalyst temperature was to further increase the gas yield and decrease the yield of oil. Total nitrogen and sulphur contents in the oils were markedly reduced such that at the catalyst temperature of 550°C a 67% reduction in nitrogen and a 56% reduction in sulphur was achieved. The pyrolysis oil contained mainly alkanes and alkenes however, after catalysis almost all of the long chain alkanes and alkenes were converted to lower molecular weight material, of the short chain, alkyl substituted and iso forms. In addition, the pyrolysis oil contained significant concentrations of single ring aromatic compounds and polycyclic aromatic hydrocarbons, consisting of mainly, benzene, naphthalene, biphenyl and phenanthrene and their alkylated derivatives. After catalysis the concentrations of these species were increased in the derived catalysed oils. The aromatic and polycyclic aromatic compounds in the oils showed a marked increase in concentration as the catalyst temperature was increased to 550°C. Nitrogen and sulphur containing aromatic and polycyclic aromatic compounds were reduced in concentration after catalysis, reflecting the overall decreases in nitrogen and sulphur contents in the oils.