Many IR spectra of complex heterogeneous substances are usually difficult to evaluate, interpret and cross-correlate. A principal component analysis (PCA) can provide additional valuable information. The scatterplot of the first and second principal components (PC) is shown in Fig. . The first two PCs explain approximately 85% of the variance of the data (scree-plot Fig. ). The four different clusters move predominantly along the 1st PC and with a small negative offset along the 2nd PC. The loadings weights (Fig. ) describe the influence of the individual wavenumbers on this displacement. Especially samples with high absorption below 1200 cm -1 are shifted into a positive direction along PC 1, but also the absorption in the carbonyl region plays a role. Along PC 1, a shift due to increasing degree of oxidation can certainly be read. In addition, the short milling times (5 min) along PC 1 hardly differ from the original sample. Only on PC 2 it becomes clear that especially high absorption values in the range around 1750-1685 cm -1 cause a shift in a negative direction. The degree of oxidation of the lignin is also reflected along PC 2.