Endocarps of the olive fruit were subjected to short-time hydrothermal carbonization (SHTC) to study the effect of temperature and holding time on the properties of hydrochars. In general, the increase of these variables improved the calorific value and the combustion properties of the hydrochars, and decreased their capacity to adsorb moisture and the ash content. The best hydrochar was produced at 225 degrees C for 10 min, showing maximum values of higher calorific value (23.4 MJ/kg), energy yield (74.7%), and comprehensive combustibility index (6.19 x 10(-7) min(-2) degrees C-3), higher values than those were managed working at 250 degrees C. The increase in energy efficiency at 225 degrees C at longer reaction residence times was related to repolimerization phenomena studied by mean of the analysis of the structural components of the biomass (fiber analysis), scanning electron microscopy inspection, and Fourier-transform infrared spectroscopy. Finally, the pyrolysis of the olive-fruit endocarp could be mathematically modeled, from thermogravimetric analysis, concluding that this process requires higher temperatures to remove volatile materials of the biomass, in comparison with the SHTC.