Auxiliary activity family 9 (AA9) is a group of fungal synergistic proteins belonging to the lytic polysaccharide monooxygenases. Via oxidative cleavage, AA9 facilitates cellulose hydrolysis by cellulase. However, oxidative activity of AA9 on pure xylan has not been reported yet. Previously, an AA9 from Chaetomium globosum (CgAA9) was identified and expressed in Escherichia coli. The recombinant CgAA9 exhibited synergistic effects in cellulose hydrolysis. Here, we showed that CgAA9 acts synergistically in xylan hydrolysis. When the insoluble fraction of xylan from beechwood was hydrolyzed by xylanase, CgAA9 yielded 1.3 times more reducing sugar than that produced only by xylanase, and the extent of synergism by CgAA9 was enhanced in the presence of ascorbic acid as a reducing cofactor. In addition, CgAA9 functioned on pretreated rice straws. This is the first report of the synergistic activity of AA9 with xylanase in the hydrolysis of pure xylan and xylan contained in pretreated lignocellulose. (C) 2016 Elsevier Ltd. All rights reserved.