Oil shales of different particle grain sizes were pyrolysed in a 10 cm diameter x 100 cm high fluidised-bed reactor under both a nitrogen and a steam atmosphere. The oil shares were from the Kimmeridge clay of Jurassic age from the UK. The influence of particle grain size on the yield and composition of the oil and gases was determined. The gases were analysed for H-2, CO, CO2, CH4 and other hydrocarbons to C-4. The condensed oils were analysed for their content of polycyclic aromatic compounds (PAC), including polycyclic aromatic hydrocarbons (PAH), sulphur-PAM (PASH) and nitrogen-PAM (PANH). The oil yield increased with increasing particle grain size. The derived shale oils contained significant concentrations of PAH, PASH and PANH. The concentration of PAC increased with decreasing particle grain size. The PAH consisted mainly of naphthalene, fluorene and phenanthrene and their alkylated homologues, and lower concentrations of fluoranthene, pyrene and chrysene. The PASH identified included benzothiophene and dibenzothiophene, and the PANH identified included indole, carbazole and their alkylated derivatives. Increased secondary reactions linked to the formation of PAC were indicated by the alkene/alkane ratio. The effect of steam was to significantly increase the yield of oil and also to increase the concentration of PAH, PASH and PANH in the derived shale oil compared with using nitrogen as the sweep gas.