Sugar cane bagasse and curaua acidolysis lignins were used to get a better understanding of the mechanism involved in a new chemical modification of sugar cane bagasse and curaua fibres, consisting in a selective oxidation of lignin by chlorine dioxide and reacting some of the created unsaturated units (quinones or muconic derivatives) with furfuryl alcohol (FA). The objective of the treatment was to create a fibre coating increasing compatibility between fibres and phenolic resins in composites. The lignins were reacted with chlorine dioxide to oxidize the phenolic units of the polymer and then treated with furfuryl alcohol. Weight percent gain of 14% and 10% were obtained for sugar cane bagasse and curaua, respectively. 1H and 31P NMR, as well as FT-IR results showed that bagasse lignin had more guaiacyl than syringyl units and the reverse for curaua lignin. 1H NMR of oxidized lignins revealed a decrease of the aromatic and methoxy content after the C1O2 oxidation, due to partial degradation of the macromolecule. Thermal analysis showed that sugar cane lignin decomposes at lower temperature than curaua lignin, partly due to the high content of condensed structural units present in curaua lignin. Condensed units decompose at higher temperatures than uncondensed ones. The reaction of oxidized lignin with FA shifted the decomposition exotherm to lower temperature for both curaua and sugar cane bagasse lignins, due to the modification introduced into their structures.