Chloroethenes such as tetrachloroethene (PCE) and trichloroethene (TCE) are among the most prevalent contaminants in groundwater due to their extensive use in industrial processes. In situ bioremediation (ISB) is an attractive technology for removal of these compounds. It relies on an anaerobic process in which specialized bacteria can obtain energy for growth using chloroethenes as electron acceptor via dehalorespiration. Engineered bioremediation is achieved by stimulating these microorganisms through the addition of electron donor in the subsurface. This technology has been widely used for bioremediation of chloroethenes plumes and recent studies have indicated promising results for bioremediation of chlorinated solvents source zone. However, application of source zone ISB is still a significant technical challenge. One of the main issues is the groundwater acidification due to dehalorespiration and fermentation processes thereby inhibiting the activity of dehalogenating microorganisms. The main objective of this work was to study the influence of pH on dechlorination rate and on bacterial community composition. To do so, anaerobic microcosms were constructed and pH was maintained constant at different values ranging from 5 to 7.5 with different buffers. These microcosms were inoculated with enriched dehalogenating consortia and were fed with PCE and hydrogen. Four different dehalogenating consortia, able to transform PCE to ethene, were tested. Dechlorination rates were determined by measurements of chloride and chloroethenes concentration. Specific PCR was used to detect known dehalogenating strain and relative changes in microbial community composition were evaluated by TRFLP. Pattern of pH inhibition varied for each dehalogenating consortium. The most resistant consortium was able to dechlorinate PCE up to pH 5. Results showed also that each step of the dechlorination presents a different sensitivity, the dechlorination of lower chlorinated ethenes being more sensitive to acidic pH. Transformation of PCE to DCE occurs up to pH 5, dechlorination of DCE to VC up to pH 5.5 and the last step, from VC to ethene up to pH 6. Analyses of community composition by TRFLP showed that the genus Dehalococcoides seems to be more sensitive to pH than other dehalogenating strains present in the consortia. These results emphasize the need to implement efficient pH control strategy to achieve complete removal of PCE in chloroethenes ISB.