Techniques for detecting and quantifying anaerobic transformations of benzene, toluene, ethylbenzene, and xylene (BTEX) are needed to assess the feasibility of using in situ bioremediation to treat BTEX-contaminated groundwater aquifers. Deuterated surrogates of toluene (toluene-d8) and xylene (o-xylene-d10) were injected into BTEX-contaminated aquifers during single-well push-pull tests to monitor for the in situ formation of deuterated benzylsuccinic acid (BSA-d8) and o-methyl-BSA-d10. Test solutions (250 L) containing toluene-d8 (4 - 22 M) and o-xylene-d10 (4 - 9 M) along with a conservative bromide tracer (1.3 mM) and nitrate (4 mM) as an electron acceptor were injected into four wells at two sites (Figure 1). Detection of BSA-d8 and o-methyl-BSA-d10 in groundwater samples collected from the same wells following injection unequivocally demonstrated anaerobic in situ toluene-d8 and o-xylene-d10 transformation with calculated zero-order formation rates ranging from 1.0 to 7.4 nM/day (Figure 2). Concurrent utilization of co-injected nitrate was rapid in all tests at both sites, with zero-order rates ranging from 13 to 39 M/hr. The field tests conducted in this study represent the first reported use of deuterated aromatic hydrocarbons to detect and quantify anaerobic BTEX transformation product formation in the subsurface.