Biological materials, rich in hemicelluloses (birch and pine wood, wheat straw and rice hulls) were disintegrated batchwise in a flow reactor with pure water at elevated temperatures and pressures. The resulting extractives and residual fibre compounds were characterized. Because of rapid removal of carbohydrate based extractives from the hot reaction zone, the degradation of pentosans and hexosans to furfural and hydroxymethylfurfural could be limited to only a small amount. Contrary to normal prehydrolysis conditions employing a sealed batch reactor, temperatures of 190 to 210°C and residence times of goods of 60 min are the optimum. Thus, hemicelluloses are dissolved and isolated with up to more than 90% yield. At temperatures above 225 "C severe destruction of hemicelluloses and solvolysis of ce&lose starts. Depending on disintegration temperatures, pentosan average molecular weights (4) vary in case of birch wood between 3 800 at 150°C and 900 at 225 "C. m e content of monomers rises with temperature and reaches a maximum of 45% at 225 "C. DP, of residual cellulose is also temperature dependent and vanes in case of birch wood and wheat straw between 1500 and 450. Lignin, which is mostly not dissolved, can be removed almost quantitatively by further treatment of the residue with ethanol/water at 250 "C, thus allowing the isolation of relatively pure cellulose.