A considerable effort has been invested in the design and synthesis of transition-state analogue inhibitors of glycosidases, particularly with respect to their potential as therapeutics. However, their characterization as transition-state mimics is generally based only upon their high affinity. To provide a quantitative evaluation of the mimicry afforded by two commonly used classes of inhibitors, a set of 10 active site mutants of the Cellulomonas fimi xylanase Cex was generated, and kinetic parameters for substrate hydrolysis as well as for inhibition by a set of 5 putative transition-state mimics were determined. Excellent linear free energy relationships (r = 0.97-0.99) were seen between binding of the inhibitors and transition-state affinities for aza-sugar inhibitors with sp(2)-hybridized "anomeric" centers, while lesser, but still significant, correlations were seen for sp(3)-hybridized inhibitors. Future design and elaboration of both classes of compounds as bespoke glycosidase inhibitors should be carried out with these findings clearly in mind.