The initial metabolic reactions for anaerobic benzene biodegradation remain uncharacterized. Isotopic data for carbon and hydrogen fractionation from nitrate-reducing, sulfate-reducing, and methanogenic benzene-degrading enrichment cultures and phylogenic information were used to investigate the initial reaction step in anaerobic benzene biodegradation. Dual parameter plots of carbon and hydrogen isotopic data (deltadelta2H/ deltadelta13C) from each culture were linear, suggesting a consistent reaction mechanism as degradation proceeded. Methanogenic and sulfate-reducing cultures showed consistently higher slopes (m = 29 +/- 2) compared to nitrate-reducing cultures (m = 13 +/- 2) providing evidence for different initial reaction mechanisms. Phylogenetic analyses confirmed that culture conditions were strictly anaerobic, precluding any involvement of molecular oxygen in the observed differences. Using published kinetic data, we explored the possibility of attributing such slopes to reaction mechanisms. The higher slopes found under methanogenic and sulfate-reducing conditions suggest against an alkylation mechanism for these cultures. Observed differences between the methanogenic and nitrate-reducing cultures may not represent distinct reactions of different bonds, but rather subtle differences in relative reaction kinetics. Additional mechanistic conclusions could not be made because kinetic isotope effect data for carboxylation and other putative mechanisms are not available.