Comprehensive two-dimensional gas chromatography-time-of-flight-mass-spectrometry can be used to resolve and identify individual petroleum-derived hydrocarbons in unresolved complex mixtures (UCMs), such as those accumulated by mussels (Mytilus edulis). Mussels exhibiting a range of scope for growth values were collected from sites around the UK coast. Tissue extracts from mussels exhibiting impaired health contained large amounts of aromatic hydrocarbon UCMs compared to the extracts from healthy mussels. The UCMs (up to 125 microg g(-1) dry tissue) contained thousands of previously unidentified branched alkyl homologues of known aromatic hydrocarbons such as branched alkylbenzenes (BABs), tetralins (BATs), and indanes and indenes (BINs). The toxicity of few such alkyl branched compounds has been investigated previously, but here we show that a commercial mixture of BABs (C12-C14) is toxic to mussels in laboratory tests (11-57 microg g(-1) dry tissue), reducing feeding rate by up to 40% in 72 h. Thus, some, if not all aromatic UCMs, apparently comprise potent mixtures of persistent, bioaccumulative and toxic compounds which have been overlooked previously.