Contamination of the subsurface environment at the Libby Superfund Site, Montana, includes polycyclic aromatic hydrocarbons and f1p4achlorophenol due to accidental spills and improper disposal of wood preserving wastes. Biodegradation is a treatment technology gaining wide application in the treatment of hazardous waste sites. A microcosm study was conducted to evaluate the effect of temperature, sampling depth, nutrient addition, and oxygen on the biodegradation potential of phenanthrene and pentachlorophenol in aquifer samples using radiolabeled chemicals. Mineralization of phenanthrene reached 14% but was less than 1% for pentachlorophenol over the 56 day incubation period. Phenanthrene mineralization in microcosms at 10 degrees C was not significantly different from those at 20 degrees C. This may have been due to microbial community acclimation to lower temperatures at the site. Average volatilization was less than 2% for both phenanthrene and pentachlorophenol. After 56 days, most of the radiolabeled chemical was either solvent extractable or soil bound.