Unresolved complex mixtures (UCMs) and polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in sediment originating from oil leaks, shipping, and other human activities and thus it is necessary to understand the role of UCM on sediment toxicity and PAH bioaccumulation. In the current study, lethal and sublethal effects of sediment-associated UCM were examined in two benthic invertebrates (Chironomus dilutus and Lumbriculus variegatus) using two spiked sediments. Results showed that UCM alone was toxic to the organisms and its toxicity was species-dependent. Approximately 1% of UCM in sediment caused 50% mortality in C. dilutus, which indicated UCM at environmentally relevant concentrations can directly cause sub-lethal and lethal effects to benthic invertebrates. Moreover, bioaccumulation testing of sediment-associated PAHs to L variegatus showed that the addition of UCM to sediment at low concentration (0.01%) increased PAH bioavailability. These findings were further confirmed by assessing bioavailability using Tenax extraction. In contrast, high concentrations of UCM in sediment (0.5%) may have formed non-aqueous phase liquids, which served as an alternative sorption phase for PAHs and reduced PAH bioavailability. Understanding the role of UCM in the overall oil toxicity and its impact on other contaminants would improve risk assessment of sediments impacted by petroleum products in the future. (C) 2011 Elsevier B.V. All rights reserved.