Abstract Phanerochaete chrysosporium is the best studied organism with respect to lignin degradation, but its degradation of the xylan component of lignocellulose is only now being studied. When grown on oat spelt xylan (mainly arabinoxylan), it produces an enzyme with β- d-xylosidase and β- d-glucosidase activity. This enzyme was purified by ultrafiltration followed by ammonium sulphate precipitation, anion-exchange chromatography using DEAE Biogel and Mono Q, and gel filtration using Superose 12. It is extracellular, with an apparent M r value of 44500 as determined by SDS-PAGE; the pI is 4.67 and activity is maximal at pH 5 and 60°C. The enzyme is of particular interest because its principal activity is against laminaribiose (3- O-β- d-glucopyranosyl- d-glucopyranose and laminarin [(1 → 3)-β- d-glucan with ca. 3% of β-(1 → 6) branches] rather than cellobiose and xylobiose. It was competitively inhibited by d-glucono-1,5-lactone and deoxynojirimycin; with p-nitrophenyl β- d-xylopyranoside as substrate, the K i values were 35 and 87.5 μM, respectively, and with p-nitrophenyl β- d-glucopyranoside, they were 35 and 68.7 μM, respectively. The K m values with p-nitrophenyl β- d-xylopyranoside and p-nitrophenyl β- d-glucopyranoside as substrates were 3.51 and 5.30 mM, respectively.