The dechlorinating activity of a methanogenic granular sludge from a methanol-fed upflow anaerobic sludge blanket reactor was investigated with chlorinated ethanes. This unadapted methanogenic consortium degraded all chloroethanes tested. The product formation rates decreased with the number of chlorine substituents. The more highly chlorinated ethanes were also converted, although at a lower rate, in the presence of autoclaved (dead) sludge, indicating the involvement of reduced heat-stable cofactors like vitamin B-12 and F-430. Direct chemical dechlorination of hexa-, penta- and tetrachloroethanes was also observed in medium without sludge, although at a much lower rate. The results show the importance of cometabolic and abiotic (chemical) conversions for the transformation of chlorinated ethanes by the methanogenic consortium, The types of reaction and the products formed were correlated with the Gibbs free-energy change (Delta G(0＇)). Reductive hydrogenolysis and dichloroelimination were important dechlorinating mechanisms. Generally these reactions have a higher Delta G(0＇) value than dehydrochlorination reactions, which occurred less frequently during the transformation of chloroethanes by the methanogenic granular sludge.