The adsorption of a purified 20.7 kDa thermostable endo-1-4-β-xylanase (EC 3.2.1.8) from a Bacillus sp. on wheat straw at 4°C was studied. Adsorption data fitted the Langmuir-type adsorption isotherm with the maximum amount of adsorbed xylanase being 521 μg protein g(-1) straw. Adsorption of the xylanase on straw, lignin, and insoluble xylans was irreversible at 4°C. The extent of hydrolysis was quantified by the measurement of total neutral sugars liberated from wheat straw-xylanase complexes at 60°C. Maximum hydrolysis was observed using 350 μg enzyme g(-1) straw and reached 11% of the xylans in the straw after 5 h of reaction. No proportionality could be found between the level of xylanase adsorption on straw and the extent of hydrolysis at 60°C. Adsorption and hydrolysis experiments indicated that all the bound xylanase was not hydrolytically active. This suggested that nonspecific adsorption occurred on lignin. Analysis of the end products of the reaction indicated that xylose and neutral and uronic acid-containing xylo-oligosaccharides were the major compounds.