The natural phenolic compounds syringaldehyde and vanillin were compared to the synthetic mediators 1-hydroxybenzotriazole, violuric acid and promazine in terms of boosting efficiency in a laccase-assisted biobleaching of eucalyptus kraft pulp. Violuric acid and 1-hydroxybenzotriazole revealed to he the most effective mediators of the bioprocess. Nevertheless, laccase-syringaldehyde system also improved the final pulp properties (28% delignification and 63.5% ISO brightness) compared to the process without mediator (23% and 61.5% respectively), in addition to insignificant denaturation effect over laccase. The efficiency of the biobleaching process was further related to changes in non-conventionally used optical and chromatic parameters of pulp, such as (L-center dot), chroma (C-center dot) and dye removal index (DRI) showing good correlation. Adverse coupling reactions of the natural phenolic mediators on Pulp lignin were predicted by electrochemical studies, demonstrating the complexity of the laccase-mediator reaction on pulp. (C) 2008 Elsevier Ltd. All rights reserved.