Hemicelluloses were extracted from the hardwood Populus tremuloides via alkaline solubilization using a modified Clextral twin-screw extruder Extraction yields were studied as a function of temperature and sodium hydroxide input flow rate, and compared to batch experiments. Based on the content of pentosans, the twin-screw reactor allows extraction of up to 90% of the initial hemicelluloses, the liquid/solid ratio being six times less than for a batch reactor and the solid reaction time being much lower Moreover an efficient liquid/solid separation was obtained, even at high caustic solution flow rate, the extracted solid being drier than the starting material. At a fixed solid flow rate (2.2 kg of dry wood/h) the solubilization of hemicelluloses was directly correlated to the specific mechanical energy (SME) absorbed by the processed material. When solubilized in water extracted hemicelluloses exhibit a thickening behavior the solution being pseudoplastic and thixotropic. Copyright (C) 1996 Elsevier Science Ltd.