Laboratory microcosms were used to determine the concentrations of oxygen (O₂) and methane (CH₄) that optimize trichloroethylene (TCE) biodegradation in sediment and ground-water samples from a TCE-contaminated aquifer at Picatinny Arsenal, Morris County, New Jersey. The mechanism for degradation is the cometabolic activity of methanotrophic bacteria. The laboratory data will be used to support a field study designed to demonstrate the effectiveness of combining air sparging with cometabolic degradation of TCE for the purpose of aquifer remediation. Microcosms were constructed in autoclaved 250-mL (milliliter) amber glass bottles with valves for repeated headspace sampling. Equal volumes (25 mL) of sediment and ground water, collected from a depth of 40 feet, were added. TCE was added to attain initial aqueous concentrations equal to the field level of 1,400 mu g/L (micrograms per liter). Nine microcosms were constructed with initial headspace O₂ concentrations of 5%, 10%, or 14% and CH₄ concentrations of 0.5%, 3%, or 5%, with nitrogen making up the balance. Sterile controls, controls without CH₄, and controls without sediment were also constructed. A 4-mL gas sample was removed periodically and TCE, O₂ , CH₄ , and carbon dioxide (CO₂) concentrations were measured by using gas chromatography. As biodegradation proceeded, the decrease in O₂, CH₄ , and TCE concentrations and the production of CO₂ were monitored. An initial acclimation period of at least 100 days was observed in those microcosms in which significant microbial activity occurred, as determined from decreases in O₂ and CH₄ concentrations and an increase in CO₂ content. Degradation of TCE occurred with O₂ concentrations of 2.7 to 8.7% and CH₄ concentrations of 0.5 to 3.5%. Microcosms that initially contained 10% O₂ and 3% CH₄ showed the greatest microbial activity and the greatest amount of TCE degradation. The greatest rates of TCE degradation occurred when O₂ and CH₄ headspace concentrations reached levels of 7.7 to 8.7% and 1.7 to 2.7%, respectively, which correspond to aqueous concentrations of 2.9 to 3.5 mg/L and 0.4 to 0.6 mg/L, respectively. Over these ranges, TCE degradation rates ranged from 15 to 20 mu g of TCE per kilogram of sediment per day. Analysis of the control microcosms indicated that these TCE degradation rates are much greater than those attributable to experimental variation. The results indicate that the microbial community of the sediment is capable of TCE degradation and that significant rates of degradation can be achieved with obtainable O₂ and CH₄ concentrations.