AbstractThe Eucalyptus genus plays an important role in the worldwide forest industry, with highly productive plantations supplying high-quality raw material for pulp and paper, wood, and biomass that would otherwise come from native forests. Lignin and extractives are important components for wood structure and protection but they are disruptive elements with respect to some industrial processes involving paper, pulp, and biomass production. This work evaluated effects of supplementation of flavonoids on the wood composition of Eucalyptus grandis x Eucalyptus urophylla (E. urograndis), a commercial hybrid. The wood samples were analyzed for extractives and lignin contents by wet chemical analysis, and the composition of lignin monomers and the carbohydrate hexosan/pentosan ratio were determined by analytical pyrolysis. The results showed that supplementation with the flavonoids naringenin and naringenin-chalcone led to an overall reduction of the extractive content and altered the monomeric composition of lignins towards a higher syringyl content. Thus, the treatment of Eucalyptus with flavonoids results in the improvement of wood quality for technological purposes.