In an attempt to find a thermostable xylanase enzyme for potential application in the pretreatment prior to H₂O₂ bleaching of paper pulp for industry, an extracellular xylanase from Aureobasidium pullulans CBS 135684 was purified 17.3-fold to apparent homogeneity with a recovery yield of 13.7%. Its molecular mass was approximately 72 kDa as determined by SDS-PAGE. The optimal pH and temperature for activity of the purified enzyme were pH 6.0 and 70°C, respectively. The enzyme was relatively stable at 50°C, retaining more than half of its original activity after 3-h incubation. The thermostability of the enzyme was improved by the addition of 0.75 mM sorbitol prolonging the enzyme’s activity up to 10-fold at 70°C. When the potential of using the enzyme in pretreatment of rice straw pulp prior to bleaching was evaluated, the greatest efficiency was obtained in a mixture containing xylanase and sorbitol. Treatment of the rice straw pulp with xylanase prior to treatment with 10% (v/v) H₂O₂ and production of hand sheets increased the ISO sheet brightness by 13.5% and increased the tensile and tear strengths of the pulp by up to 1.16 and 1.71-fold, respectively, compared with pulps treated with H₂O₂ alone. The results suggested the potential application of the enzyme before the bleaching process of paper pulp when the maintenance of high temperature and enzyme stability are desirable.