In this study two different microbial cultures (one constructed with lagoon sediments from an industrial area in Venice, Italy and the other with aquifer material from a chlorinated solvent-contaminated site in Rho, Italy) were investigated for their ability to dechlorinate perchloroethylene (PCE), 1,1,2,2-tetrachloroethane (TeCA) and their mixtures with butyrate as the primary electron donor (i.e., the H-2-releasing substrate) in the presence and in the absence of sulfate, an ubiquitous groundwater component. In all the tested conditions, the presence of sulfate had a detrimental effect on the rate of reductive dechlorination. This effect was most likely due to the rapid and competitive utilization of H-2 by the sulfate-reducing populations present in the cultures. Indeed, in all sulfate-containing setups a "steady-state" dissolved H-2 concentration in the range of 0.7-2.9 nmol L-1 was typically observed during butyrate degradation. At such low H-2 levels the dechlorination process was rate-limited by the electron donor availability. However, in spite of the fact that in all the sulfate-containing setups the H-2 level was nearly the same (i.e., 0.7-2.9 nmol L-1) the extent of reduction of dechlorinating activity greatly depended on the culture tested and the chlorinated compound administrated (from 18% to over 95% reduction of initial dechlorination rate). The results of this study indicate that the extent of competition for H2 between sulfate-reducers and dechlorinators, and in turn the impact of an electron donor addition at a contaminated site, are strongly dependent on the relative kinetics Of H-2-releasing and H-2-consuming reactions and cannot be predicted a priori simply based on differences in H-2 thresholds concentration values among competitive metabolisms. (c) 2007 Elsevier Ltd. All rights reserved.