Nonadapted freshwater sediment slurries and sediment slurries adapted to dechlorinate 2,3-dichloropyridine (2,3-Cl(2)Pyd), 2,3-dichloroaniline (2,3-Cl(2)Anl), 2,3-dichlorophenol (2,3-Cl(2)Pyd), 3,5-dichloropyridine (3,5-Cl(2)Pyd), 3,5-dichloroaniline (3,5Cl(2)Anl) and 3,5-dichlorophenol (3,5-Cl(2)PhOH) were studied to determine the rate, range and extent of biotransformation of structurally related compounds under anaerobic conditions. 2,3-dichloroanisole (2,3Cl(2)Ans) and 3,5-dichloroanisole (3,5-Cl(2)Ans) were initially demethylated, producing 2,3-Cl(2)PhOH and 3,5Cl(2)PhOH as intermediate transformation products. All other dichloroaromatic compounds examined were initially dechlorinated. The rates of dechlorination of 2,3-Cl(2)PhOH, 2,3-Cl(2)Anl, and 2,3-Cl(2)Pyd were significantly lower (5-15 times) in nonadapted sediment slurries compared to sediment slurries adapted to 2,3-Cl(2)Anl or 2,3-Cl(2)Pyd. In 2,3-Cl(2)PhOH adapted sediment, the rate of dechlorination of 2,3-Cl(2)PhOH was 15 times greater than in nonadapted sediment; however, the rates of dechlorination of 2,3-Cl(2)Anl and 2,3-Cl(2)Pyd were similar for 2,3-Cl(2)PhOH-adapted and nonadapted sediment slurries. In adapted and nonadapted sediment slurries, 2,3-Cl(2)PhOH, 2,3-Cl(2)Anl, and 2,3-Cl(2)Pyd were preferentially dechlorinated at the ortho, meta, and meta positions, respectively. Additionally, 2,3-Cl(2)Pyd adapted sediment slurries dechlorinated 2,3-Cl(2)PhOH and 2,3Cl(2)Pyd at both ortho and meta positions. Rates of dechlorination of 3,5-Cl(2)PhOH, 3,5-Cl(2)Anl, and 3,5-Cl(2)Pyd were lower (2-4 times) in nonadapted sediment slurries compared to sediment slurries adapted to 3,5-Cl(2)Anl or 3,5-Cl(2)Pyd. In 3,5-Cl(2)PhOH adapted sediment, the rate of dechlorination of 3,5Cl(2)PhOH was approximately 10 times greater than in nonadapted sediment. In contrast, rates of dechlorination of 3,5-Cl(2)Anl and 3,5-Cl(2)Pyd were similar in 3,5Cl(2)PhOH-adapted and nonadapted sediment slurries. A single meta chlorine was removed for all 3,5-dichloroaromatic compounds tested except 3,5-Cl(2)Ans, which was initially demethylated. These results illustrate differences in the specificity and cross-reactivity of microbial populations adapted to structurally related dichloroaromatic compounds.