The effects of four aeration and four organic loading (OLR) rates on trichloroethylene (TCE) degradation in methanogenic-methanotrophic coupled reactors were studied using ethanol as the carbon source for the methanogens. Microcosm and PCR studies demonstrated that methanotrophs capable of mineralizing TCE and methanogens were present in the biomass throughout the study. The gene for the particulate form of methane monooxygenase (pMMO) was detected by PCR, but not that for the soluble form (sMMO). TCE mineralization by methanotrophs was therefore due primarily to pMMO activity. Low TCE concentrations were measured in effluent and off-gas samples in all cases. Volatilization losses were 0-5%. Dichloroethylene (DCE) was also observed, but vinyl chloride and ethylene were never detected. Changes in the aeration rate had no effect on TCE removal, but did influence DCE degradation. Reductive dechlorination of TCE to DCE was favored at low and no-aeration conditions, and DCE accumulation occurred due to slow DCE degradation. Low DCE levels were observed at the higher aeration rates, which indicated that conditions in these reactors were amenable to the aerobic co-metabolism of TCE and DCE. The OLR did have an effect on TCE removal. TCE and DCE removal were negatively affected when the OLR was increased. An OLR of 0.3 g COD 1(rx)(-1)day(-1) or lower with an aeration rate of 3 1(02) l(rx)(-1)day(-1) and higher is the recommended operating condition of a coupled reactor for removal of TCE.