To increase the amount of oxygenated functional groups that contribute to the hydrophilicity of a flexible carbon surface, hydrothermal carbonization was applied. A 20 wt% slurry of Pseudotsuga menziesii was hydrothermally treated at 200 250 degrees C for 0.5 24 h in the presence of 0 100 g of zinc chloride (ZnCl2). Hydrochar yield, residual cellulose ratio, H/C, O/C, and the amount of oxygenated functional groups (OFGs) were evaluated. The amount of OFGs decreased due to dehydration of cellulose and other components, but increased due to hydrolysis of molecular structures such as lactone ring-opening caused by prolonged reaction time and by addition of ZnCl2. These studies revealed that ZnCl2 was an effective additive to promote carbonization and to increase OFGs formation at 250 degrees C in hydrothermal carbonization of woody biomass.