CO2 and CH4 radiocarbon and stable carbon isotope ratios were used to assess natural attenuation at a fuel-contaminated soil site at the Norfolk Navy Base, Norfolk, VA (USA). Soil gas samples were collected spatially over a monitoring network in October 2002 and in March 2003. CO2 and CH4 from regions with high petroleum concentrations were C-depleted relative to uncontaminated areas. C-depleted methane suggested methanogenic hydrocarbon degradation. The difference in CO2 age between background and plume-influenced areas indicated that approximately 90% of the CO2 at the latter was petroleum derived, making contaminant the primary source of carbon for the microbial assemblage.