The action of xylanases on chemical weed pulps was studied in the presence of a 1 mM HgCl2solution. In that medium, the encc-cellulases present in the crude enzyme mixture from the Easidicmycete Sporotrichum dimorphosporum were inhibited as shown by soluble sugar analysis end by Molecular weight determination of the carbanilated derivatives of the enzyme-treated pulps. After enzymatic hydrolysis, although the mass loss was less than2%of the dry material weight, important structural modifications were revealed by physical property determinations on the residual pulps. The water retention value which increases by20%, the mean pore radius which is reduced by a factor of 10, the scanning electron microscopic photographs which show fiber flexibility and external fibrillation and the molecular weight distribution curves, allow us to come to the conclusion that xylans are actually hydrolyzed in the whole delignified cell walls. Therefore, because of the xylan distribution with regard to cellulose microfibrils, the selective hydrolysis by xylanases might affect the cell wall cohesion and, thus, influence the papermaking properties of pulps.