In the wavelength range of 3500-1850 cm -1 there are two vibration bands of interest (Fig. ). On the one hand, the range of the O-H stretching vibration (1) from 3500-3000 cm -1 and on the other hand the different -C-H stretching vibrations (2). The absorption of the O-H stretching vibration decreases slightly in the range of 3300-3000 cm -1 compared to the unmodified Kraft lignin. This shoulder of the main signal represents associated hydroxyl groups The reason for this is probably that on the one hand OH functional groups are oxidised to the carbonyl (-C=O) and possibly condensation reactions of OH groups occur. The release due to the high pH value (approx. 12) and the oxidation of hydroxyl groups are probably in direct competition with each other. The different C-H stretching vibrations show a clear increase of the signals at 2945 cm -1 (-CH3-) and 2880 cm -1 (-CH2-) (S1). Presumably, this is caused by demethoxylation, which leads to the increase of arylalkyl chains by methyl migration. Another explanation could be the depolymerisation of the macromolecule, leading to smaller lignin fragments, which in turn have more terminal or sidechain -CH3-groups. The intensities at 2840 cm -1 show that the C-H vibration of the -O-CH2/3 unit remains largely unchanged. However, the lack of shoulder intensity makes interpretation difficult.