Xylanase A (XynA) from Clostridium stercorarium, which consists of a catalytic domain and two family VI cellulose-binding domains (CBDs) each connected by a Linker sequence, was found to bind to insoluble oat-spelt xylan as well as acid-swollen cellulose (ASC). Its adsorption to xylan was greatly dependent on the concentration of the phosphate buffer in the binding assay mixtures. The adsorption of XynA to insoluble xylan proceeded in accordance with a Langmuir-type isotherm. The Ka and [PX](max) values of XynA for oat-spelt were 0.17 l/mu mol and 5.0 mu mol/g, respectively. Removal of the CBDs from XynA abolished the cellulose-and xylan-binding abilities of this enzyme and reduced the enzyme activity toward insoluble xylan but not soluble xylan. These results clearly indicated that the CBDs of XynA play an important role in the hydrolysis of insoluble xylan. Desorption of XynA from the ASC-XynA complex was effected by soluble saccharide solutions such as barley beta-glucan, birchwood xylan, and glucose solutions as well as a cellobiose solution, indicating that the CBDs of XynA have an affinity for soluble saccharides in addition to insoluble polysaccharides.