In this project the toxicity of the unresolved complex mixtures (UCMs) from water-soluble fractions (WSFs) of petrogenic oils were determined by trout hepatocyte bioassays. The WSFs of two weathered crude oils, evaporated to remove light compounds with boiling points < 200°C (Troll 200°C+ and Statfjord 200°C+ residues) were separated in non-polar to polar fractions by preparative high-pressure liquid chromatography (HPLC) and individual fractions analysed in the bioassays. Toxic responses were primarily associated with one polar fraction (fraction 11) rich in UCM. Chemical characterization of this fraction by Time of Flight Mass spectrometry (GCxGC-TOF-MS) identified many isomeric sulphur hetero-aromatics including a range of aromatic sulph-oxides.Typical identifiable semi-soluble compounds in the WSFs, like decalins, naphthalenes, polycyclic aromatic hydrocarbons (PAH) and alkylated phenols, did not seem to contribute to the toxicity of this fraction. In addition, typical UCM-related compounds were toxic at low concentrations in the bioassays.