AbstractWheat straw (Triticum aestivum L.) and oat straw (Avena sativa L.) were subjected to acid and alkaline pre-treatments partly in combination with hydrogen peroxide. The aim was to remove lignin and increase the accessibility of the polysaccharides to enzymatic digestion. Accessibility was evaluated by digestion with a cell wall degrading enzyme complex to yield reducing sugars that may serve as precursor substrates for biofuels or building block chemicals. Changes in lignin, hemicelluloses, as well as amorphous, semi-crystalline, and crystalline regions of cellulose moieties of pretreated straw were efficiently characterized by Fourier transform near-infrared (FT-NIR) reflectance spectroscopy. These alterations of the chemical structure of straw after different pre-treatment methods were powerfully differentiated by principal component analysis (PCA). Characteristics of the different samples owing to the different pre-treatment methods could be clustered from the PCA loadings spectra.