The demonstration of monitored natural attenuation (MNA) of chlorinated hydrocarbons in groundwater is typically conducted through the evaluation of concentration trends and parent-daughter product relationships along prevailing groundwater flow paths. Unfortunately, at sites contaminated by mixtures of chlorinated ethenes, ethanes, and methanes, the evaluation of MNA by using solely concentration data and parent-daughter relationships can result in erroneous conclusions regarding the degradation mechanisms that are truly active at the site, since many of the daughter products can be derived from multiple parent compounds. Stable carbon isotope analysis was used, in conjunction with concentration data, to clarify and confirm the active degradation pathways at a former waste solvent disposal site where at least 14 different chlorinated hydrocarbons have been detected in the groundwater. The isotope data indicate that TCE, initially believed to be present as a disposed product and/or a PCE dechlorination intermediate, is attributable to dehydrochlorination of 1,1,2,2-PCA. The isotope data further support that vinyl chloride and ethene in the site groundwater result from dichloroelimination of 1,1,2-trichlorethane and 1,2-dichloroethane, respectively, rather than from reductive dechlorination of the chlorinated ethenes PCE, TCE, or 1,2-DCE. The isotope data confirm that the chlorinated ethanes and chlorinated methanes are undergoing significant intrinsic degradation, whereas degradation of the chlorinated ethenes may be limited. In addition to the classical trend of enriched isotope values of the parent compounds with increasing distance associated to biodegradation, shifts of isotope ratios of degradation byproduct in the opposite direction due to mixing of isotopically light byproducts of biodegradation with compounds from the source are shown to be of high diagnostic value. These data underline the value of stable isotope analysis in confirming transformation processes at sites with complex mixtures of chlorinated compounds.