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Measurements of the stable carbon isotope ratios (δ¹³C) of microbial metabolic end products presents a promising method for monitoring in situ bioremediation of petroleum hydrocarbons. Differences between the δ¹³C values of hydrocarbons and indigenous carbon sources (e.g., plant matter, soil carbonates) can be exploited to trace the origins of metabolic end products. However, in zones of methanogenesis and/or where the δ¹³C values of endogenous plant matter overlap those of hydrocarbons, δ¹³C measurements can produce ambiguous results. In such cases, simultaneous measurement of the radiocarbon (¹⁴C) contents of metabolic end products can be used to determine their sources. This method was applied at a gasoline station spill site where hydrocarbons were the only source of ¹⁴C-free carbon. Combined δ¹³C and ¹⁴C measurements of soil gas CO₂ and dissolved inorganic carbon in …