A fractured sandstone aquifer at an industrial site in southern California is contaminated with trichloroethene (TCE) and cis- 1,2-dichloroethene (cis-DCE) to depths in excess of 244 m. Field monitoring data suggest thatTCEis undergoing reduction to cis- DCE and that additional attenuation is occurring. However, vinyl chloride (VC)andethenehavenotbeendetected in significant amounts, so that if transformation is occurring, a process other than reductive dechlorination must be responsible. The objective of this study was to evaluate the occurrence of biotic and abiotic transformation processes at this site for TCE, cis- DCE, and VC. Anaerobic microcosms were constructed with site groundwater and sandstone core samples. 14Clabeled compounds were used to detect transformation products (e.g., CO2 and soluble products) that are not readily identifiable by headspace analysis. The microcosms confirmed the occurrence of biotic reduction of TCE to cis-DCE, driven by electron donor in the groundwater and/or sandstone. VC and ethene were not detected. Following incubation periods up to 22 months, the distribution of 14C indicated statistically significant transformation of [14C]TCE and [14C]cis-DCE in live microcosms, to as high as 10% 14CO2 from TCE and 20% 14CO2 from cis- DCE. In autoclaved microcosms, significant transformation of [14C]TCE and [14C]cis-DCE also occurred; although some 14CO2 accumulated, the predominant 14C product was soluble and could not be stripped by N2 from an acidic solution (referred to as nonstrippable residue, or NSR). Characterization of the NSRbyhigh-performanceliquidandionchromatographyidentified glycolate, acetate, and formate as significant components. These results suggest that a combination of abiotic and biotic transformation processes is responsible for attenuation of TCE and cis-DCE in the fractured sandstone aquifer. Tracking the distribution of 14C during the microcosm study was essential for observing these phenomena.