Bio-oil obtained from fast pyrolysis of lignocellulosic biomass usually contains up to 30% of pyrolytic lignin that mainly consists of phenolic oligomers. Phenolic oligomers are difficult to upgrade into useful products since they are non-volatile and viscous, and highly reactive. The present study investigates the origin of phenolic oligomers during fast pyrolysis. Lignin was pyrolyzed in a micropyrolyzer and the vapor was directly analyzed using online GC/MS, or recovered in a cold solvent then analyzed using GPC and high-resolution mass spectrometry. A total of 569 phenolic compounds with molecular weight less than 504 Da were found, which are significantly smaller than phenolic oligomers recovered from condensed bio-oil. The most abundant phenolic compounds among these smaller molecules are monomers followed by dimers, trimers and tetramers. Pyrolysis of model compounds at identical experimental conditions showed that primary phenolic monomers with high reactivity, for example, 2-methoxyl-4vinylphenol, syringol and guaiacol, could reoligomerize during fast pyrolysis via secondary reactions. It was also found that phenylcoumaran, stilbene and biphenol structures are relatively stable during fast pyrolysis. Therefore the dimers containing these structures could also be primary products of lignin during fast pyrolysis. The study suggests that a significant fraction of phenolic oligomers present in bio-oil arise from reoligomerization of small phenolic molecules formed during the pyrolytic depolymerization of lignin. (C) 2014 Elsevier Ltd. All rights reserved.