The application of the laccase mediator system (L stage) to TCF bleached pulp from Eucalyptus globulus with a low residual lignin content provides useful information about its effect in the hexenuronic acids content, functional groups (carboxyl and carbonyl) and electrokinetic properties as ζ potential and surface charge. The use of laccase from Trametes villosa in combination with the mediator 1- hydroxybenzotriazole (HBT) produces an oxidation of the cellulose to carbonyl groups, and reduced the amount of carboxyl groups present in TCF pulp through partial removal of hexenuronic acids from it. This result can open up new prospects for increasing brightness stability. Additionally, the laccase mediator system modifies the surface charge and ζ potential of the fibre suspension for the removal de ionizables groups in TCF pulp. The reduction of the carboxyl content additionally alters the charge on fibre surfaces; this may be useful with a view to reducing pulp processing costs since a decreased content in ionizable groups can be expected to reduce consumption of cationic additives used in the production of paper. As regards electrokinetic properties, the laccase mediator system reduces the ζ potential of fibre suspensions by acting on carboxylic and hexenuronic acids. The biomodification of the chemical composition of TCF pulp by the laccase mediator system causes no substantial changes in the properties of the refined pulp (ºSR, %WRV); this indicates that the L treatment does not affect development of the primary effects of refining, and also that it may have a side effect on the fibre-water system by decreasing the proportion of ionizable groups and altering the electrokinetic properties of the pulp