Batch experiments were conducted to assess both the biotransformation potentials of one hydrocarbon (alpha-pinene) and four alcohol monoterpenes (arbanol, linalool, plinol, and alpha-terpineol) under nitrate-reducing conditions at 23 degrees C, as well as their effects on the nitrate-reducing process. A mixed, nitrate-reducing culture developed from a forest-soil extract was enriched using ethanol as the electron donor and used in this study. a-Pinene was not biotransformed under the conditions of this study and inhibited both ethanol and nitrate utilization. Partial transformation of the alcohol monoterpenes was observed and resulted in inhibition of the nitrate-reducing process and cessation of further utilization of the added monoterpenes. Accumulation of biotransformation products - mainly hydrocarbon monoterpenes such as camphene, beta-myrcene, and d-limonene - was observed. The hydrocarbon monoterpenes formed may have been responsible for the observed inhibition of the nitrate-reducing process and lack of complete utilization of the alcohol monoterpenes. These results have significant implications for the expected rate and extent of biotransformation of monoterpenes under anoxic conditions as well as their effect on the nitrate-reducing process in both engineered and natural systems.