A highly functionalized carbonaceous material (hydrochar) was obtained by means of the hydrothermal carbonization (250 degrees C) of two representative types of biomass, i.e. eucalyptus sawdust and barley straw. This product has a brown colour; it contains around 50-60% of the carbon originally present in the biomass and it is composed of particles that retain the cellular appearance of the raw material. These particles are covered by microspheres (1-10 mu m) which were probably formed as a consequence of the transformation of the cellulose fraction. From a chemical point of view, the hydrochar products have a high degree of aromatization and they contain a large amount of oxygen-containing groups (i.e. carbonyl, carboxylic, hydroxyl, quinone, ester, etc) as was confirmed by Raman, IR and XPS spectroscopic techniques. The presence of these oxygen functionalities on the surface of the hydrochar particles explains their high water affinity (hydrophilic properties). On the basis of the highly condensed chemical nature of the hydrochar products, we postulated that this material has a recalcitrant nature that could lead to a significant increase in carbon turnover time in relation to the biomass. This suggests an important route for the sequestration of CO2 present in the atmosphere. (C) 2011 Elsevier Ltd. All rights reserved.