The synthesis of xylobiose-derived nitrogen-containing inhibitors of xylanase is described starting with accessible precursors through efficient synthetic schemes. Four disaccharides were identified as powerful competitive inhibitors of the retaining family 10 xylanase, Cex, from Cellulomonas fimi, namely imidazole (K-i = 150 nM), lactam oxime (K-i = 370 nM), isofagomine (K-i = 130 nM), and deoxynojirimycin (K-i = 5800 nM) derivatives of xylobiose. By contrast, none of the compounds inhibited the family 11 xylanase, Bcx, from Bacillus circulans, to an appreciable extent. Two possible explanations are provided for the discrimination exhibited by the imidazole and the lactam oxime. One explanation relates to the different active site locations of the acid/base residue in the two enzymes: anti to the C1-O5 bond for the family 10 Cex and syn for the family 11 Bcx. The other explanation concerns proposed differences in the transition. state conformation for the two enzymes: half-chair for Cex and boat for Bcx. The reasons for the difference in inhibition values between Cex and Bcx for the isofagomine and deoxynojirimycin derivatives are less clear-cut but may be ascribed to destabilizing steric and electrostatic interactions between the inhibitors and an essential tyrosine residue in the active site of Bcx.