The pyrolysis of biomass with catalyst upgrading was studied in a combined fluidised-bed pyrolysis unit with online fixed-bed zeolite catalytic upgrading; the biomass was in the form of mixed wood waste, and the catalyst used was zeolite HZSM-5. The influence of pyrolysis and catalyst conditions on the formation of polycyclic aromatic hydrocarbons (PAH) was investigated. There is much interest in PAH in the environment, since among the major chemical groups PAH comprise the largest group of carcinogens. Process parameters investigated included pyrolysis and catalyst temperature, the influence of catalyst ageing and regeneration, and vapour residence time over the catalyst. It was found that polycyclic aromatic hydrocarbons in biomass pyrolysis oils were in low concentration; however, after catalyst upgrading there was a marked rise in PAH, and their concentration reached high levels. The concentration of PAH increased with catalyst temperature. The freshly activated catalyst gave the highest concentrations of PAH, and as successive regenerations took place the catalyst became progressively less active, producing lower levels of PAH. Extended run-time with the catalyst also resulted in a less active catalyst and lower concentrations of PAH. Shorter vapour residence times produced lower concentrations of PAH. The pyrolysis oils after catalysis contained high concentrations of certain PAH which are of known mutagenic and/or carcinogenic activity; for example the oils contained tri- and tetra-methylphenanthrenes, chrysene, methylchrysenes and benzopyrenes. The influence of high PAH concentrations in a hydrocarbon product that may be used as a fuel are discussed. Work has shown a direct link between fuel PAH concentrations and PAH concentrations in emissions from both diesel engines and furnaces. The PAH concentration in petroleum-derived fuels are much lower than those found in zeolite catalytically upgraded biomass pyrolysis oils.