The high tendency of bio-oil towards polymerization is one of the key challenges in hydrotreatment of bio-oil for production of bio-fuels. However, such properties of bio-oil could be used to produce carbon materials, but the mechanism for the polymerization has not been clarified. Bio-oil contains sugars, derivatives of sugars (i.e. furans et al.), phenolic monomers and oligomers. Among of them, the furans and phenolics are the typical compounds in bio-oil. In this study, the potential cross-polymerization between the furans (furan, furfural and furfuryl alcohol (FA)) and the phenolics (vanillin and guaiacol) in bio-oil was explored in water with/without H2SO4. The furans could polymerize via ring-opening to produce reactive reaction intermediates. Furfural and FA contained branched functionalities in furan ring, which was more reactive towards polymerization, especially for furfuryl alcohol. In general, the furans were more reactive towards polymerization than vanillin and guaiacol. Nevertheless, the cross-polymerization between the furans and phenolics occurred and significantly enhanced its tendency towards polymerization. The reaction intermediates formed via opening of the furan ring were the key initiator for the cross-polymerizations. Additionally, the presence of H2SO4 could enhance consumption of the C=O during the polymerization and the thermal stability of the resulting polymer. (C) 2020 Elsevier Ltd. All rights reserved.