Petroleum hydrocarbons and other related organic constituents are the most common contaminants found in groundwater. People commonly speak of substances such as gasoline, diesel and other fuels, and solvents, as if they are very familiar with them and clearly understand what they are and how they behave in the subsurface. The truth of the matter is that these substances are very complex mixtures of many organic chemicals, each defined by its own respective physical and chemical properties. It is these specific properties of the original petroleum that allow one to characterize or fingerprint its refined products. The unique circumstances that exist in the formation of petroleum make it quite unlikely for any two distinct petroleum sources (releases) to share the exact same chemistry. In conducting subsurface assessmentand remediation-related activities, it thus becomes important to have a fundamental understanding of the chemistry of these substances, and thus their relative behavior in the environment. It is therefore important at many sites with complex histories and commingled contaminant NAPL pools and dissolved hydrocarbon plumes to differentiate and fingerprint such occurrences spatially and temporally.