Hydrothermal carbonization (HTC) is an important thermochemical process where biomass is converted into coal-like solid products known as hydrochars. The HTC process is performed in hot-compressed water under self-generated pressures. In this work, the HTC of acorn shells was conducted at various temperatures and reaction times with and without boric acid (H3BO3). A high degree of carbonization occurred at 250 degrees C in non-catalytic and catalytic 4-h runs. Hydrochars obtained from non-catalytic and catalytic runs at 250 degrees C consisted of spherically carbon particles with diameters ranging from 303 nm to 3.27 mu m. Carbon spheres at 200 and 225 degrees C for 2 h were not observed. The yield and carbon content of the hydrochars were slightly increased by reuse of the process water in thermal runs without catalysts.