Four novel disazo dyes were synthesized with a hydroxyl group in the para position of the phenolic ring in relation to the disazo bond, whereas the other substituents on the phenolic ring varied. The decolorization of the dyes was studied using a purified fungal laccase. Dyes I to III were not substrates for the phenoloxidase laccase regardless of whether or not redox mediators were added. In contrast, rapid and total decolorization of dye IV was obtained. Laccase oxidation of dye IV was described using Michaelis-Menten kinetics, giving an apparent affinity constant of 0.5 mM and a V-max of 2.76 mM min (-1). N-hydroxybenzotriazole had no effect on the laccase catalysis whereas violuric acid inhibited the decolorization of disazo dye IV. A hypothetical mechanism is proposed for the biotransformation of this dye by laccase. (C) 2002 Elsevier Science Ltd. All rights reserved.