The detection of natural estrogenic compounds, including 17α-estradiol (αE2), 17β-estradiol (βE2), and estrone (E1), in surface water has ignited concern because of their potential to disrupt endocrine function, activate hormone responses, and alter secondary sex characteristics in aquatic species at low environmental concentrations. Once in surface water, sediments can act as a source or a sink for these estrogenic compounds. Little is known, however, about the transformation processes of αE2, βE2, and E1 in sediments under anaerobic conditions. The purpose of this study was to characterize the relative biotransformation rates and compounds formed for these estrogens under the full range of redox conditions present in agro-impacted ditch/stream sediment. Specifically, our goals were to: (i) ascertain if inter-conversion occurs between the parent compounds (αE2 and βE2) under anaerobic conditions; (ii) determine if the primary metabolite (E1) transforms back to its precursors (αE2 and βE2) and under what redox conditions; and (iii) determine if the E2 isomers degrade at the same rate under both aerobic and anaerobic conditions. Anaerobic batch microcosms under controlled redox conditions were amended with αE2, βE2, or E1, and stored in the dark in an anaerobic chamber until time of sacrifice. Electron acceptor concentrations and methane production were analyzed on a Seal AQ-2 and Agilent 7890A Gas Chromatograph and microcosms re-amended as needed. Sterile controls were used to discern between biotic and abiotic processes. Microcosms were analyzed for the parent compounds and known metabolites on an LC/MS/MS. Preliminary results from iron-reducing and methanogenic experiments will be compared to our previously reported results under nitrate-reducing and sulfate-reducing conditions.  These previous studies showed: (i) inter-conversion between αE2 and βE2, likely through E1 as the intermediate; (ii) reversible transformation from the primary metabolite E1 to both parent compounds, with preferential transformation back to βE2; and (iii) longer half-lives for the alpha isomer than βE2 under both redox conditions. The results-to-date indicate that once these hormones partition from the water into the anaerobic sediment bed, E1 may transform to αE2 and βE2, which are often the more potent contaminants of concern.