The detection of significant levels of halogenated aliphatic contaminants in groundwater resources in the United States has spurred a considerable effort to understand the various mechanisms--both microbiological and abiotic--by which these compounds may be transformed. In aerobic environments, the abiotic reactions that predominate are elimination of hydrogen halide (dehydrohalogenation) and nucleophilic substitution by H2O (hydrolysis). Little research has examined the rates and pathways of abiotic reactions that may be significant under hypoxic conditions. The dehalogenation of 1,2-dichloroethane (1,2-DCA) and 1,2-dibromoethane (EDB) in phosphate buffer was examined alone, and in phosphate buffer containing hydrogen sulfide. Kinetic runs were conducted at pH 7 over the temperature range from 25 to 87.5 C. Phosphate buffer catalyzed the hydrolysis of both 1,2-SCA and EDB. This catalysis was attributed to the buffer anion HPO4(2-). The rate laws for these transformations exhibit the following form: -d(RX)/dt=(K(sup 1)(sub H) + K(sub HS(-))(HS(-)) + K(sub HPO4(2-)(HPO4(2-))(RX) where RX denotes either 1,2-DCA or EDB, and the rate constants correspond to the overall reactions.