The deep burrowing, invasive spionid polychaete, Marenzelleria spp. (3 sibling species), is rapidly expanding its range in the Baltic Sea ecosystem, increasing the depth of the bioturbated zone dramatically relative to the native benthic community. One concern is the effect of this invasion on the fate of deeply buried sediment-associated pollutants in the historically polluted Baltic Sea. We report here the results of an experimental microcosm study examining the fate of 14C radiolabelled pyrene (a 4-ring PAH) in sediment microcosms with and without Marenzelleria viridis. We also investigated the impact of depositing labile organic matter (seston) on the sediment surface. Worms clearly enhanced the release of pyrene and degradation metabolites from the sediment to the overlying water in all cases, mostly due to the initial flushing of sediments during burrow establishment. Surprisingly, there was no clear effect of worms on the mineralization of pyrene to CO2, where a complicated interaction between bioturbation and the presence of labile organic matter was observed. Furthermore, the ultimate fate of the increased release of sediment-associated pyrene and its metabolites to the water is unknown.