Biological reductive dechlorination can be used to remediate aquifers contaminated with chlorinated solvents. This process can be stimulated with the addition of hydrogen as an electron donor. A small field study was conducted in a trichloroethene (TCE)-contaminated aquifer at the Twin Cities Army Ammunition Plant site in Arden Hills, MN, to determine whether hollow-fiber membranes could be used to deliver hydrogen to the contaminated groundwater to stimulate bioremediation in situ. Nonporous siliconecoated membranes were placed in Geoprobe wells, where they were able to sustain hydrogen delivery over 20 months without a decline in gas transfer performance. High concentrations of hydrogen (168 Ϯ 13 M) were observed in the wells containing membrane modules. These concentrations were similar to that predicted using a clean water gas transfer gas correlation (255 M). High acetate concentrations (662 Ϯ 172 M) were also observed in the wells containing membrane modules during hydrogen addition. The addition of hydrogen via the hollow-fiber membranes stimulated vinyl chloride and ethene formation, likely as a result of the activity of acetotrophic dechlorinators.