Chlorinated solvents are among the most common industrial contaminants of groundwater. Tetrachloroethene (PCE) is resistant to aerobic biodegradation; however, it is occasionally reductively dechlorinated in anoxic contaminated aquifers. This is particularly true if the subsurface also contains other organic compounds that can serve as electron donors and whose utilization by subsurface bacteria will deplete the available oxygen. Chlorinated solvents are followed closely in importance as groundwater contaminants by alkylbenzenes. Alkylbenzenes most commonly enter the subsurface in the form of fuel spills and are degraded under both aerobic and anaerobic conditions by subsurface microorganisms. Freedman and Gossett have shown that a number of low molecular weight organic compounds can serve as electron donors for reductive dechlorination, and recently, Scholz-Muramatsu et al. demonstrated that benzoate can also serve as a source of reducing equivalents for reductive dechlorination of PCE. Grbic-Galic and Vogel have shown that benzoate is an intermediate of anaerobic toluene degradation. Overall these findings suggest that certain alkylbenzenes, such as toluene, could potentially serve as electron donors for the reductive dechlorination of PCE. Field studies conducted by this laboratory have suggested that this may indeed be occurring at field sites exposed to both alkylbenzenes and chlorinated solvents. In the study, the biologically mediated interactions of toluene and PCE under anaerobic conditions were investigated by using microcosms constructed with aquifer solids from an area that was exposed to both alkylbenzenes and chlorinated ethenes at the U.S. Coast Guard Air Station, in Traverse City, MI. (Copyright (c) 1991 American Chemical Society.)