Fast pyrolysis followed by catalytic hydroconversion is a value chain aimed to transform lignocellulosic biomass into biofuel or chemicals. During hydroconversion, desired catalytic deoxygenation reactions are in competition with thermal side reactions like condensation or oligomerization. These undesired pathways lead to high molecular weight compounds (i.e. macromolecules) that are responsible for catalyst deactivation and severe plugging of the reactor. We investigate here the impact of a phenolic compound on the formation of these macromolecules. Catalytic hydroconversion of a fast pyrolysis bio-oil and a bio-oil/guaiacol (50/50 wt%) mixture were carried out in a batch reactor using a NiMo/alumina catalyst. An extended analytical strategy has been developed involving size-exclusion chromatography (SEC) and liquid state C-13 NMR dedicated to the in depth characterization of effluents as well as physicochemical analysis of the fresh and used catalyst (XRD, Hg porosimetry, N-2 physisorption, STEM). This strategy allowed bringing new insights on aromatic structures larger than 1000 g.mol(-1) and their formation mechanism. This formation can be chemically inhibited by the introduction of organic component such as guaiacol. This stabilization was mainly observed and explained at low temperature and short reaction time.