Hexenuronic acids (HexA) of hemicellulosic heteroxylan were shown to play important role in brightness development of chemical pulps during xylanase-aided bio-bleaching. Industrial wood (eucalypt) kraft Pulp and a few non-wood (giant reed) organosolv pulps were pre-treated with commercial xylanase preparations (endo-1,4-beta-xylanase activity; EC 3.2.1.8) and bleached by simplified bleaching sequence. The HexA performance was examined and compared with control (enzyme-free) samples. The xylanase-assisted direct brightening effect, noted immediately after an enzymatic stage, was proved to be caused by exclusive HexA removal with solubilized HexA-carrying xylooligosaccharide fractions. Aldo- hexa- and aldopentahexenuronic acids (Xyl(5)-HexA and Xyl(4)-HexA) were found as predominant oligosaccharides, accounting for up to 65% of total acidic oligomers in enzymatic hydrolyzates. A strong positive correlation (R-2 = 0.91) was established between further brightness improvement of xylanase-treated pulps during subsequent chemical bleaching (bleach boosting) and the content of HexA. This underlined the role of HexA as one of the key factors in definition of final brightness of bio-bleached pulps,determining to a large extent the bleaching efficiency of xylanase application as a whole. (C) 2009 Elsevier Ltd. All rights reserved.