Microbial endoxylanases reduce chemical demand when they are applied to pulps prior to bleaching sequences, However, their mechanism of action is not fully understood, and the criteria for determining which enzymes are most effective are not well known. We examined the effects of two types of well-characterized endoxylanases from Streptomyces sp. TUB B-12-2 on the bleachability of kraft softwood and hardwood pulps, xyl 1a is a low molecular mass, high pI xylanase that forms mainly xylotriose but no xylose from oat spelts xylan. xyl 3 is a high molecular mass, low pI xylanase that forms mainly xylose and xylobiose as end products. xyl 3 released more reducing sugars than did xyl 1a, but both enzymes released about the same amount of total sugars. High performance liquid chromatography analysis of product sugars from pulp showed that at pH 7.0, xyl 1a formed oligosaccharides with a high degree of polymerization, but at pH 9.0 the action patterns of the two enzymes were similar. xyl 3 released 12% more chromophores from hardwood and 30% more from softwood pulp than did xyl 1a. Both enzymes enhanced brightness while reducing chemical demand, but xyl 3 was more effective. Synergism was noted in the prebleaching of softwood but not hardwood pulp. Both enzymes were active at pH 9, and the presence of pulp increased thermal stability.