The reductive dechlorination of tetrachloroethene (PCE) and trichloroethene (TCE) at contaminated sites often results in the accumulation of lesser chlorinated products, (primarily cis-1,2-dichloroethene, DCE, and vinyl chloride, VC) rather than the more desirable and non-toxic end product, ethene. The accumulation of chlorinated intermediates is generally ascribed to a lack of Dehalococcoides strains that can dechlorinate DCE and VC, electron donor limitations, and/or reaction kinetics. However, competition among multiple dehalorespiring populations for electron acceptors and/or donors can also potentially affect the extent of PCE dechlorination in situ. Competitive interactions may be particularly important if both Dehalobacter restrictus (Dhb) and Dehalococcoides ethenogenes (Dhc) are present because both organisms utilize the same electron donor (H2) and respire PCE and TCE. However, Dhb cannot dechlorinate PCE beyond DCE, whereas Dhc is capable of dechlorinating PCE to ethene. The objectives of this study were to: (1) obtain unique estimates of Monod kinetic parameters (including the maximum specific substrate utilization rate, qmax, and halfsaturation coefficient, Ks) for chlorinated ethene and hydrogen utilization by these two strains, and (2) independently estimate inhibition constants to describe inhibition among the chlorinated ethenes. Kinetic parameters were fit using non-linear methods to data obtained in batch assays under conditions that were carefully controlled to yield mathematically meaningful results. Under PCE-limiting conditions, Dhb has a higher qmax, PCE (22.5 μmol/mg VSS/h) and lower Ks, PCE (7.2 μM) compared to Dhc (qmax, PCE, 6.8 μmol /mg VSS/h and Ks,PCE, 22.5 μM) . These results suggest that under electron acceptor-limiting conditions characteristic of natural attenuation treatment scenarios, Dhb might outcompete Dhc for PCE (and TCE). Unless hydrogen is available downgradient for Dhc to use as an electron donor while respiring DCE, these results could explain why DCE often accumulates at field sites. In addition, dechlorination of VC by Dehalococcoides was strongly inhibited by even very low concentrations of PCE, which could also impede the complete transformation of PCE to ethene in situ. The results of these kinetic parameter determinations will be used in conjunction with mathematic models to make more refined predictions of the interactions, growth and functions of Dhb and Dhc, and thus the rate and extent of PCE transformation, in plumes that contain both organisms.