Biomass in the form of pine wood was pyrolysed in an externally heated 7.5 cm diameter, 100 cm high fluidised bed pyrolysis reactor with nitrogen as the fluidising gas. A section of the freeboard of the reactor was packed with zeolite ZSM-5 catalyst. The pyrolysis oils before and after catalysis were collected in a series of condensers and cold traps. In addition, gases were analysed off-line by packed column gas chromatography. The compositions of the oils and gases were determined in relation to the primary fluidised bed and after catalysis at increasing catalyst bed temperatures from 400 degrees to 550 degrees C. The oils were analysed by a number of techniques to determine composition, including liquid chromatography, gas chromatography/mass spectrometry, Fourier transform infrared spectroscopy and size exclusion chromatography. The results showed that the oils before catalysis were highly oxygenated; after catalysis the oils were markedly reduced in oxygenated species with an increase in aromatic and polycyclic aromatic species. The gases evolved from the fluidised bed pyrolysis of biomass were CO2, CO, H-2, CH4, C2H4, C3H6 and minor concentrations of other hydrocarbon gases. After catalysis the concentrations of CO2 and CO were increased. The conversion of oxygenated compounds was mainly to H2O at lower catalyst temperatures and CO2 and CO at high catalyst temperatures. Detailed analysis of the oil showed that there were high concentrations of biologically active polycyclic aromatic species in the catalysed oil which increased with increasing catalyst temperature. The oxygenated compounds in the uncatalysed oil were mainly phenols and carboxylic acids. After catalysis these decreased in concentration with increasing catalyst temperature.