Holm oak (Quercus ilex) sapwood was submitted to a hydrothermal treatment and liquid fractions rich in hemicelluloses were obtained. The chemical composition as well as the molar mass were analysed during the experiments. The effect of temperature (130-170 degrees C) and reaction time (5-220 min) on the conversion and molar mass of the obtained hemicelluloses was investigated. The results were compared to previously published data obtained using Norway spruce. The results show that the extraction rate depends strongly on the wood species used. The maximum yield (approximately 60%) was obtained at 170 degrees C after 20 min. The molar mass of the hemicelluloses decreased during the extraction due to hydrolysis and the pH of the solution decreased as deacetylation occurred simultaneously. Temperature influenced significantly the hydrolysis rate of the macromolecules. Compared to Norway spruce (softwood), the average molar mass in Holm oak (hardwood) was lower under the same reaction conditions. (C) 2017 Elsevier B.V. All rights reserved.