Compound-specific analysis of stable carbon and hydrogen isotopes was used to assess the fate of the gasoline additive methyl tert-butyl ether (MTBE) and its major degradation product tert-butyl alcohol (TBA) in a groundwater plume at an industrial disposal site. We present a novel approach to evaluate two-dimensional compound-specific isotope data with the potential to identify reaction mechanisms and to quantify the extent of biodegradation at complex field sites. Due to the widespread contaminant plume, multiple MTBE sources, the presence of numerous other organic pollutants, and the complex biogeochemical and hydrological regime at the site, a traditional mass balance approach was not applicable. The isotopic composition of MTBE steadily changed from the source regions along the major contaminant plume (-26.4parts per thousand to +40.0parts per thousand (carbon); -73.1parts per thousand to +60.3parts per thousand (hydrogen)) indicating substantial biodegradation. Constant carbon isotopic signatures of TBA suggest the absence of TBA degradation at the site. Published carbon and hydrogen isotope fractionation data for biodegradation of MTBE under oxic and anoxic conditions, respectively, were examined and used to determine both the nature and the extent of in-situ biodegradation along the plume(s). The coupled evaluation of two-dimensional compound-specific isotope data explained both carbon and hydrogen fractionation data in a consistent way and indicate anaerobic biodegradation of MTBE along the entire plume. A novel scheme to reevaluate empiric isotopic enrichment factors (E) in terms of theoretically based intrinsic carbon ((12)k/(13)k) and hydrogen ((1)k/(2)k) kinetic isotope effects (KIE) is presented. Carbon and hydrogen KIE values, calculated for different potential reaction mechanisms, imply that anaerobic biodegradation of MTBE follows a S(N)2-type reaction mechanism. Furthermore, our data suggest that additional removal process(es) such as evaporation contributed to the overall MTBE removal along the plume, a phenomenon that might be significant also for other field sites at tropic or subtropic climates with elevated groundwater temperatures (25 degreesC).