The purpose of this study was to explore the differences of the hemicelluloses extracted by hydrothermal pretreatment using water and alkaline solutions(Na OH or KOH). The physicochemical properties and structural characteristics of two water-soluble and four alkali-soluble hemicelluloses extracted from the triploid of Populus tomentosa Carr. through the hydrothermal pretreatment were comparatively studied. It was observed that the alkalis(Na OH and KOH) were more effective than distilled water as extractants. Sugar analysis showed that xylose(66.83%~86.49%) was the major constituent, followed by glucose(6.83%~18.49%). Mannose(1.40%~8.42%), galactose(2.17%~4.05%), and arabinose(0.21%~2.26%) were also detected in the hemicellulosic fractions. The results of gelpermeation chromatography(GPC) indicated that the hemicelluloses extracted using the alkaline solutions had relatively higher molecular weights than those solubilized in distilled water. Further, based on spectroscopic ~1Hnuclear magnetic resonance(~1H-NMR) and two-dimensional heteronuclear singular quantum correlation(2D-HSQC) analyses, it was confirmed that the hemicellulosic fractions had a major structure of(1→4)-b-D-xylan and a minor structure of(1→4)-a-D-glucan with small amounts of substituted sugars and glucuronic acid attached. The purpose of this study was to explore the differences of the hemicelluloses extracted by hydrothermal pretreatment using water and alkaline solutions (NaOH or KOH). The physicochemical properties and structural characteristics of two water-soluble and four alkali-soluble hemicelluloses extracted from the triploid of the Populus tomentosa Carr. through the hydrothermal pretreatment were comparatively studied. It was observed that the alkalis (NaOH and KOH) were more effective than distilled water as extractants. Sugar analysis showed that xylose (66.83% -86.49%) was the major constituent , followed by glucose (6.83% ~ 18.49%). Mannose (1.40% ~ 8.42%), galactose (2.17% ~ 4.05%), and arabinose (0.21% ~ 2.26%) were also detected in the hemicellulosic fractions gel permeation chromatography (GPC) indicated that the hemicelluloses extracted using the alkaline solutions had relatively higher molecular weights than those solubilized in distilled water. Further, based on spectroscopic ~ 1Hnuclear magnetic resonance (~ 1H-NMR) and two-dimensional heteronuclear singular quantum correlation (2D-HSQC) analyzes, it was confirmed that the hemicellulosic fractions had a major structure of (1 → 4) -bD-xylan and a minor structure of 1 → 4) -aD-glucan with small amounts of substituted sugars and glucuronic acid attached.