Bioremediation can be promoted as an in situ technique for cleaning groundwater by amending the groundwater with selected nutrients and substrates to stimulate microbial activity in the aquifer. It is presumed in doing this that there pre-exists microbial activity, of the appropriate kind, in the aquifer. Frequently, if the appropriate bacteria are present in an aquifer, pollutants will show signs of natural attenuation as a matter of course, so the question arises, is there any value in nutrient supplements? An experiment performed at the C.F.B. Borden test site compared two isolated 24 m long by 2 m wide by 4 m deep sections of aquifer exposed to a mixture of carbon tetrachloride, tetrachloroethene (PCE), and toluene (TOL), migrating at 10 cm/day. In one section, the control, the compounds were permitted to naturally attenuate for 1 year. Over the same time period, the three test compounds in the second section were subjected to sequenced treatment involving first anaerobic conditions and then aerobic conditions. The design of this experiment permits a rare assessment of the effects of nutrient additions compared with natural attentuation under nearly identical conditions. All three injected compounds showed indications that they were naturally degrading in the control section at the end of one year, on the basis of flux measurements. In the treatment section, the redox extremes were supported by benzoate and oxygen amendments, respectively. The degradation rates of PCE and TOL were significantly increased as a result of the chemical additions, also based on flux measurements. There were strong indications that the same was true of the intermediate compounds trichloroethene and cis-1,2 dichloroethene. In all cases treatment distances were reduced and lag periods shortened to the point that treatment was nearly complete within the 24 m section at the end of the experiment; much longer treatment distances were required for natural attenuation. These data document, in a well controlled comparative field experiment, that even where natural attenuation is occurring benefits can accompany nutrient amendments.