Stable hydrogen-isotope values of ground water (δ₂H) and dissolved hydrogen concentrations (H_(2(aq)) were quantified in a petroleum-hydrocarbon contaminated aquifer to determine whether the production/consumption of H₂ by subsurface microorganisms affects ground water &delta₂H values. The range of &delta₂H observed in monitoring wells sampled (-27.8 ‰_c to -15.5 ‰_c) was best explained, however, by seasonal differences in recharge temperature as indicated using ground water δ₁₈O values, rather than isotopic exchange reactions involving the microbial cycling of H₂ during anaerobic petroleum-hydrocarbon biodegradation. The absence of a measurable hydrogen-isotope exchange between microbially cycled H₂ and ground water reflects the fact that the amount of H₂ available from the anaerobic decomposition of petroleum hydrocarbons is small relative to the amount of hydrogen present in water, even though milligram per liter concentrations of readily biodegradable contaminants are present at the study site. Additionally, isotopic fractionation calculations indicate that in order for H₂ cycling processes to affect δ₂H values of ground water, relatively high concentrations of H₂ (>0.080 M) would have to be maintained, considerably higher than the 0.2 to 26 nM present at this site and characteristic of anaerobic conditions in general. These observations suggest that the conventional approach of using δ₂H and δ₁₈O values to determine recharge history is appropriate even for those ground water systems characterized by anaerobic conditions and extensive microbial H₂ cycling.