The action of β-xylosidase from Trichoderma reesei against different substituted xylo-oligosaccharides was studied. The enzyme cleaved off all unsubstituted xylose units from the non-reducing end of 1,2-linked uronic acid substituted xylo-oligosaccharides. Surprisingly, an l-arabinofuranosyl group linked α-1,3 to the xylopyranosyl ring was found to protect the β-1,4-xylosidic linkage before the substituted xylose unit from being cleaved by the β-xylosidase. Most probably the 1,3-linked substituent sterically hinders the hydrolysis. According to the results of the present work, β-xylosidase of T. reesei is not able to remove all unsubstituted xylose units from the non-reducing end of substituted xylo-oligosaccharides, as had been believed previously.