Hydrothermal carbonization processes were studied under different conditions using two different biomass materials: walnut shell and sunflower stem. Coalification under mild conditions was promoted with the aim of increasing the calorific value of the solid hydrochar. Hydrocarbonization processes brought up an increase on the heating value of the materials up to 28.9 and 29.3 MJ kg(-1), for sunflower stem and walnut shell, respectively. which corresponds to an increase of 1.75 and 1.50 fold when compared with the natural biomass. Also, regarding the variables studied it was found that temperature and water/biomass ratio were more influent on the hydrocarbonization process than residence time. The hydrochars show negligible N-2 adsorption at 77 K and interesting surface morphology, characterized by the formation of microspheres which grow as temperature increases. Moreover, hydrochars are acid in nature and present several oxygen functionalities, as observed from FT-IR and pzc analysis. (C) 2011 Elsevier B.V. All rights reserved.