A water-soluble extracellular polysaccharide, designated Os2-1, was isolated from the fermented liquid of the fungus Oidiodendron truncatum GW using ethanol precipitation, anion-exchange and gel-permeation chromatography. Os2-1 was mainly composed of glucose with minor amounts of glucosamine, and its average molecular weight was about 9.6 kDa. On the basis of chemical and spectroscopic analyses, including one- and two-dimensional nuclear magnetic resonance (1D and 20 NMR) spectroscopy, the backbone of Os2-1 was characterized to mainly consist of (1 -> 6)-linked alpha-D-glucopyranose residues with minor amounts of (1 -> 2)-linked alpha-D-glucopyranose residues. The antioxidant activity of Os2-1 was evaluated with the scavenging abilities on 1,1-diphenyl-2-picrylhydrazyl (DPPH), superoxide radicals and inhibition effect of lipid peroxidation in vitro, and the results indicated that Os2-1 had good antioxidant activity, especially scavenging ability on DPPH and superoxide radicals. The investigation demonstrated that Os2-1was an extracellular polysaccharide differing from previously described extracellular polysaccharides, and could be a potential antioxidant. (C) 2013 Elsevier Ltd. All rights reserved.