Reductive anaerobic dehalogenation is a useful method for remediation of sites contaminated by chlorinated ethylenes, where hydrogen concentration plays the key role. Under anaerobic conditions, dehalogenating bacteria compete best against methanogenic consortia when the hydrogen level is low; and methanogenic consortia outplay dehalogenating bacteria when the hydrogen level is high. Thus, in an anaerobic mixed culture, efficient use of hydrogen for dehalogenation can be achieved by strategies that maintain hydrogen at a certain low concentration. However, due to the role of acetate, expected dehalogenating results cannot be obtained and unexpected methane formation can be encountered in practice.