The unresolved complex mixture (UCM) frequently dominates organic extracts isolated from estuarine and coastal sediments in the vicinity of industrial centers. Despite an obvious link to a petroleum source, speculation exists that biogenic sources also contribute to the UCM. To determine the source of the UCM to these environments, natural abundance radiocarbon (?(14)C) and stable carbon (?(13)C) isotopic composition of the UCM solvent-extracted from coastal sediments, road dust, and urban atmospheric particulate in the United States was measured. Extracts of UCM and separate saturate and aromatic fractions from all samples are predominantly (>90%) fossil-derived and hence have a petroleum source. Even the polar fraction of the UCM, which has a ?(14)C composition reflecting contributions from recently photosynthesized carbon (-665‰), is composed of ~66% fossil carbon indicating the presence of petroleum residues that have been transformed into more polar derivatives. The ?(13)C of the UCM had consistent values (-27.65 ± 0.51‰; n = 16) for all but one sample, indicating a common origin of the UCM. We conclude that in coastal areas dominated by human activities whole fractions of the UCM, as well as separate saturate, aromatic, and polar fractions, are principally derived from petroleum sources.