The β-xylosidase (EC 126.96.36.199) of Trichoderma reesei was purified and its characteristics and use in the hydrolysis of steamed birch xylan were studied. The enzyme was a glycoprotein with a molecular weight of 100000 as determined by SDS-gel electrophoresis and its isoelectric point was 4.7. The pH optimum was 4.0 and temperature optimum 60°C. β-Xylosidase was competitively inhibited by xylose and the inhibition constant was 2.3 mM. The purified enzyme also showed α-arabinofuranosidase activity.