Laccase of Trametes versicolor was able to oxidize in vitro most of the 14 polycyclic aromatic hydrocarbons (PAH) tested. Acenaphthylene was removed by 37% followed by anthracene and benzo[a]pyrene which were oxidized by 18 and 19%, respectively. Lower but significant oxidation of about 10% was found for-eight additional PAH : acenaphthene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, and perylene. Naphthalene, fluorene, and phenanthrene were recovered unchanged after incubation for 72 h with laccase. Addition of 1-hydroxybenzotriazole (HBT) to the reaction mixture increased oxidation of PAH : acenaphthylene, acenaphthene, fluorene, anthracene, benzo[a]pyrene, and perylene were almost completely removed from the reaction mixture. Oxidation of pyrene and benzo[a]anthracene increased from 8 and 6% without a mediator to 48 and 53% in the presence of HBT. Other PAH were nor significantly influenced by the addition of this mediator. PAH-quinones as oxidation products were formed from all PAH to different extents. A part of PAH was polymerized in the laccase/mediator system to products of weight-average molecular weight (MW) of approximately 1,500 Da. The correlation of the ionization potentials of PAH with the oxidation of these compounds is limited to the alternating PAH.