The degradation of Phenol Red dye (belonging to the phenolsulphonephthaleinic class) by laccase in the presence or absence of the mediator 1-hydroxybenzotriazole (HOBT), was investigated. Decolourisation of Phenol Red with laccase alone was limited with a maximum decolourisation degree of around 40% after 48 It. This strange behaviour is due to the mechanisms developed in this system generating intermediate compounds that formed high molecular weight materials. However, the laccase-HOBT couple with high redox potential was able to complete degradation of this dye. (C) 2003 Elsevier Ltd. All rights reserved.