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In this study, hemicelluloses of aspen wood (Pópulus trémula) were obtained by oxidative delignification in an acetic acid-water-hydrogen peroxide medium at temperatures of 70–100 °C and a process time of 1–4 h. The maximum polysaccharide yield of up to 9.68 wt% was reported. The composition and structure of the hemicelluloses were studied using a complex of physicochemical methods: gas and gel permeation chromatography, Fourier-transform infrared spectroscopy, 2D nuclear magnetic resonance spectroscopy, and thermogravimetric analysis. The xylose, mannose, galactose, and glucose monomer units were identified in the hemicelluloses by gas chromatography. The weight average molecular weight Mw of the products determined by gel permeation chromatography was found to range within 8932–33,142 g/mol. The reported Fourier-transform spectra of the hemicelluloses contain all the bands characteristic of heteropolysaccharides; a weak lignin absorption signal in the spectra at 1500–1510 cm−1 is attributed to a minor content of phenolic fragments in the structure of the obtained hemicelluloses. The use of thermogravimetric analysis established that the hemicelluloses isolated from aspen wood are resistant against heating to temperatures of up to 90–100 °C and, upon further heating up to 400 °C, start destructing at an increasing rate. The antioxidant activity of the hemicelluloses was examined using the compounds that mimic free radicals (1,1-diphenyl-2-picrylhydrazyl) and hydroxyl radicals (salicylic acid). It was found that the activity of all polysaccharides in neutralizing DPPH and hydroxyl radicals is lower than the absorption capacity of vitamin C at all the tested concentrations (0.5, 2, and 5 mg/mL) and attains 81.7 and 82.9%, respectively. [ABSTRACT FROM AUTHOR]