A competitive enzyme assay has been employed to estimate the relative binding of xylo- and cello-oligosaccharides to the active site of the Family 10 xylanase A from Pseudomonas fluorescens sp. cellulosa. The substrate used in the assay was pNPC, the hydrolysis of which can be detected colorimetrically. Consistently with the endo-xylanase nature of this enzyme, the K-i(app) for xylo-oligosaccharides decreases with increase in degree of polymerization. Cello-oligosaccharides were poorer inhibitors of pNPC hydrolysis than xylo-oligosaccharides, and the K-i(app) did not decrease as dramatically with degree of polymerization. The implications of these results are discussed in the context of other studies on xylanase A and other Family 10 xylanases.