Traditional hydrocarbon biomarker analyses determined the degree of biodegradation in two reservoir and two surface oils. These data were then correlated to the distribution and type (rings plus double bonds) of acidic NSO species selectively ionized and mass resolved by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) to determine if oxygenated polars could be used to estimate the degree of biodegradation in crude-oil contaminated soil (surface) samples. Since the histories of the surface samples were unknown (as it often the case for environmental samples), non-degraded and severely degraded reservoir oils were used as compositional benchmarks. The biodegraded reservoir crude oil exhibited an increase in relative abundance of O-2-species, a decrease in acyclic fatty acids, an increase in multi-ring naphthenic acids and a decrease in C-32 hopanoic acids compared to the non-biodegraded reservoir crude oil. The surface oils exhibited trends similar to the biodegraded reservoir oil, indicating that the surface oils were biodegraded in the reservoir, the environment or both. However, one surface sample also exhibited biomarker signatures indicative of a non-degraded oil. This evidence, in combination with the ESI FT-ICR MS data, suggests that the sample was contaminated with a mixture of biodegraded and non-degraded oils and demonstrates how analysis of hydrocarbon and polar fractions can reveal complex histories of surface contamination. This work is the first detailed compositional analysis of acidic NSO species in crude oil soil extracts and lays the foundation for our understanding of how surface biodegradation effects the composition of polars. (c) 2006 Elsevier Ltd. All rights reserved.