Four xylan rich by-products, namely wheat bran, brewery's spent grain, corn cobs and Eucalyptus wood, were characterised and subjected to a mild hydrothermal treatment in order to release and degrade the xylan from the starting materials. The chemical characterisation of the feedstock materials, with emphasis on the extracted xylan fractions and using enzymatic degradation of these xylans, resulted in rather detailed pictures of the xylans present. Depending on the feedstock material studied, the xylan present was substituted with arabinose, 4-O-methylglucuronic acid and acetyl groups. During the hydrothermal treatment, arabinose was rather easily removed from the xylan-backbone (wheat bran, brewery's spent grain and corn cobs). The acetyl groups were partly released from the feedstocks, becoming available to catalyse the depolymerisation of the xylan. Also, part of the uronic acids were released, mainly during the treatment of Eucalyptus wood. Due to the partial release of the substituents and cleavage of the xylan by the treatment performed, a wide variety of xylo-oligosaccharides with different structural features corresponding to the xylan-structure of the original feedstock were obtained. Xylo-oligosaccharides branched with arabinose were identified in the hydrolysate from brewery's spent grain, while in the hydrolysate of corn cobs and Eucalyptus wood xylo-oligosaccharides substituted with 4-O-methylglucuronic acid were present as well. Additionally, a series of partially acetylated (acidic) xylo-oligosaccharides was identified in the Eucalyptus wood hydrolysate.