Dehalococcoides species appear to be unique in their ability to grow via reductive dechlorination of dichloroethenes and/or vinyl chloride and therefore are thought to play a key role in the complete detoxification of sites contaminated with tetrachloroethene (PCE) and trichloroethene (TCE). However, heterotrophic chlorinated ethene-respiring bacteria, including certain DesulfItobacterium and Desulfuromonas species, are also frequently detected at PCE- and TCE-contaminated, sites and their growth may be promoted by the additiçn of organic electron donors at sites where biostimulation is applied. Recently, it has been shown that heterotrophic dehalorespirers may either complement Delialococcoides populations by acting as PCE-to-DCE (or TCE) respiring specialists or hinder the activity of Dehalococcoides populations by outcompeting them for chlorinated ethenes and/or reducing equivalents [Becker. 2006. Environ. Sci. Technol. 40:4473- 44801. Therefore, the presence of dehalorespiring Desulfuromonas or Desulfitóbacterium populations could have a significant effect on the in-situ bioremediation of PCE at sites where Dehalococcoides strains are naturally present or added through bioaugmentation. To understand and predict the effect of heterotrophic dehalorespirers on the fate of PCE at sites undergoing natural attenuation or engineered bioremediation, a consistent and accurate characterization of the kinetics of the known dehalorespirers is needed. In this study, the substrate utilization kinetics of two PCE dechlorinators, Desulfitobacterium strain PCE 1 and Desulfuromonas inichiganensis strain BB 1, were characterized in batch cultures. The source cultures were maintained at a constant solids retention time by feeding and wasting on a semi-continuous basis to avoid culture history effects on the estimated parameters. The ratio of initial substrate to biomass concentrations was fixed at 20 (on a chemical oxygen demand basis) to ensure that the estimated parameters are unique and reflect the intrinsic kinetics of the organisms. At relatively low chlorinated ethene concentrations, substrate utilization by the two strains can be described using a dual Monod equation. Nonlinear regression was used to fit the Monod kinetic parameters to single substrate depletion curves. The identifiability and reliability of the estimated parameters were confirmed by sensitivity analyses. In general, the chlorinated ethene half-saturation coefficient (Ks) values are higher than those reported for other PCErespiring cultures. Substrate and competitive inhibition at higher chlorinated ethene concentrations were also evaluated for Desuijitobacterium strain PCE1. An Andrews model inhibition coefficient, Ki, was fit to a profile of the initial dechlorination rate as a function of the initial PCE concentration, and little substrate inhibition was observed. The effect of TCE on the initial PCE dechlorination rates was also evaluated. TCE was found to have a weakly inhibitory effect on PCE dechlorination by fitting a competitive inhibition coefficient to the rate data. The potential implications of these kinetic parameter estimates on bioremediation practice will be discussed.