Chloroform (CF) can undergo reductive dechlorination to dichloromethane, chloromethane, and methane. However, competition for hydrogen (H-2), the electron-donor substrate, may cause poor dechlorination when multiple electron acceptors are present. Common acceptors in anaerobic environments are nitrate (NO3-), sulfate (SO42-), and bicarbonate (HCO3-). We evaluated CF dechlorination in the presence of HCO3- at 1.56 e(-) Eq/m(2)-day, then NO3- at 0.04-0.15 e(-) Eq/m(2)-day, and finally NO3- (0.04 e(-) Eq/m(2)-day) along with SO42- at 0.33 e(-) Eq/m(2)-day in an H-2-based membrane biofilm reactor (MBfR). When the biofilm was initiated with CF-dechlorination conditions (no NO3- or SO42-), it yielded a CF flux of 0.14 e(-) Eq/m(2)-day and acetate production via homoacetogenesis up to 0.26 e(-) eq/m(2)-day. Subsequent addition of NO3- at 0.05 e(-) Eq/m(2)-day maintained full CF dechlorination and homoacetogenesis, but NO3- input at 0.15 e(-) Eq/m(2)-day caused CF to remain in the reactor's effluent and led to negligible acetate production. The addition of SO42- did not affect CF reduction, but SO42- reduction significantly altered the microbial community by introducing sulfate-reducing Desulfovibrio and more sulfur-oxidizing Arcobacter. Dechloromonas appeared to carry out CF dechlorination and denitrification, whereas Acetobacterium (homoacetogen)may have been involved with hydrolytic dechlorination. Modifications to the electron acceptors fed to theMBfR caused the microbial community to undergo changes in structure that reflected changes in the removal fluxes.