To better understand the microbial degradation of chemicals in anaerobic environments, the degradation of chlorinated phenols was examined in freshwater sediments. The study demonstrated that the relative persistence of the dichlorophenols depended on the chemical structure of the individual isomers. The initial step in degradation was the reductive dechlorination of a single chlorine to form the monochlorophenol as an intermediate. Chlorine at the 2-position of dichlorophenols was the most readily reduced. The results show that caution should be used in extrapolating laboratory data to predict degradation rates at field sites.