The pyrolysis of biomass to produce bio-oil is a very effective way of biomass use. Bio-oil undergoes drastic structural changes as it is upgraded into biofuels or used as a fuel for gasification/combustion. The evolution of aromatic ring systems in bio-oil is a key consideration in bio-oil use. A bio-oil sample produced from the fast pyrolysis of mallee wood at 500 degrees C, its lignin-derived oligomers, and pure cellulose have been pyrolyzed in a novel two-stage fluidized-bed/fixed-bed reactor at temperatures between 350 and 850 degrees C. The product tars were characterized with ultraviolet (UV) fluorescence spectroscopy. Our results indicate that significant portions of aromatic ring systems in the bio-oil could turn/polymerize into solids not soluble in CHCl3 + CH3OH during the pyrolysis at relatively low temperatures, e.g., 350-400 degrees C. This process can be enhanced by the presence of cellulose-/hemicellulose-derived species in the bio-oil, which are reactive and produce radicals to enhance the polymerization reactions. The pyrolysis of cellulose-derived species in the bio-oil tended to form additional very large aromatic ring systems at temperatures higher than 700 degrees C.