The effects of 2 polychaetes, Nereis diversicolor and Arenicola marina, on the microbial mineralization of the organic contaminant pyrene, a polycyclic aromatic hydrocarbon (PAH), were followed over 44 d. We also examined whether the effect of the polychaetes was caused by enhanced oxygen supply, altered pyrene bioavailability and/or a changed abundance or activity of pyrenedegrading bacteria. The presence of polychaetes enhanced microbial pyrene mineralization by 180 to 200% compared with defaunated sediment. Collectively, the replicates of the different treatments showed that mineralization rates were positively correlated with the amount of oxidized sediment, which comprised mainly the 3 mm surface layer and zones around burrows (burrow sediment). The biogenic sediment structures had similar mineralization potential and abundance of pyrene-degrading bacteria as surface sediments. Pyrene mineralization potential in bulk (reduced and presumably anoxic) sediment was significantly lower than for surface and burrow sediments. However, when the bulk sediments were oxidized, mineralization rates increased rapidly. Collectively, these data indicate that oxygen availability controlled pyrene mineralization in these experiments. On the other hand, the presence of the polychaetes significantly reduced the bioavailability of pyrene to the microbial degraders. Pyrene bioavailability in burrow sediment was always lower than the bioavailability in both surface and bulk sediments. In addition, N. diversicolor and especially A. marina decreased the bioavailability of pyrene in surface sediments compared with that of surface sediments in the non-bioturbated control. In conclusion, these polychaetes enhanced microbial pyrene mineralization significantly and this enhancement seemed to be caused by the increased oxygen supply due to burrow construction and irrigation. In contrast, these worms decreased pyrene bioavailability and, hence, counteracted to some extent the stimulating effect of irrigation.