A twin-screw extruder equipped with a filtration module was employed for alkaline extraction of poplar hemicelluloses. Experimental designs were employed to study the influence of sodium hydroxide concentration, temperature, liquid and solid flow rates and screw rotation speed or the yield of extracted hemicelluloses. The effects on these parameters on the specific mechanical energy and the residence time distribution in the liquid phase were examined. The sodium hydroxide concentration was found to have a strong influence on the consistency of the reaction mixture, defining two effective zones of operation for extraction of hemicelluloses in the twin-screw extruder. Extraction yield was shown to depend markedly on temperature, while reduction in screw rotation speed and solid flow rate affected the mean residence time (MRT) of the liquid phase, which enhanced the extraction yield of hemicelluloses.