This work provides an assessment on the hydrothermal processing (or autohydrolysis treatments) of two lignocellulosic wastes: mixed herbs (denoted MH, mainly belonging to the Lolium species), and sunflower seed shells (denoted SS). Compositional data were obtained for both raw materials (which contained cellulose, heteroxylan and Klason lignin as major components). In autohydrolysis experiments, the raw materials presented a "susceptible fraction" which was solubilized according to a first-order kinetics. Hemicellulose degradation was followed by determination of xylooligomers, xylose and xylan substituents (arabinosyl, uronic acid and acetyl substituents). Kinetic modelling of hemicellulose degradation was carried Out considering four consecutive, first-order, pseudohomogeneous reactions. The molecular weight distribution of polymeric and oligomeric fractions derived from xylan was assessed from High Performance Size Exclusion Chromatography data. (C) 2009 Elsevier Ltd. All rights reserved.