Two Eucalyptus globulus and one Pinus pinaster kraft pulps were submitted to laccase-mediator system (LMS) to evaluate the effect of the treatment on biodelignification and subsequently on hexenuronic acid content (HexA). As expected, the hexenuronic acid content of unbleached E. globulus kraft pulps is substantially higher than the corresponding values for unbleached R pinaster pulp. Besides, the two unbleached E. globulus pulps revealed different HexA content, having an important effect the raw material. The amount of HexA is quite high in the unbleached clone E. globulus (64.1 mmol/kg), while it is moderately high in the industrial E. globulus kraft pulps (52.1 mmol/kg). These values represent between 29.1 and 38.5% of kappa number of these pulps. For unbleached R pinaster kraft pulps, the contribution of hexenuronic acid to the pulp kappa number was considerably lower. The three unbleached pulps were submitted to an optimized laccase-violuric acid treatment and the results have shown that the delignification extent for hardwood and softwood were different, but also different for the same species. For E. globulus, it was observed that 49.3% of kappa number has reduced for the clone eucalypt, while 42.2% of kappa number reduction was observed for the industrial eucalypt pulp, with the same laccase-mediator system conditions. On the other hand, laccase-mediator system removes 35.9% of the initial kappa number of the unbleached softwood pulp. From the standpoint of the low delignification extent, the amount of lignin removed was almost twice. The effect of laccase-mediator system is mostly on lignin and the hexenuronic acid decrease slightly in the pulps. (C) 2007 Elsevier Inc. All rights reserved.