Generally different anaerobic degradation , toluene, ethylbenzene and xylene isomers (BTEX) has been reported due to site specific as the indigenous microbial population, electron acceptors (EA) available and concentrations of each BTEX compound. During the 1 and 2 WWTs, biological heterotrophic dissimilative NO denitrification was confirmed by simultaneous detection of both NO and NO and significant production of CO during the NO degradation. The biodegradation fractions of NO injected during the 1 and 2 WWTs were 1.7 and 5.0%, respectively, with 7.18 and 8.85 mmol N/L/day of in situ zero-order denitrification rate coefficients. The concentrations of benzene, ethylbenzene, and xylenes measured were similar to values calculated when considering only dilution, but the measured concentrations of toluene were significantly lower than the values calculated were. These results indicate that in situ method presented in the study successfully evaluate anaerobic biodegradation potential of individual BTEX compounds by indigenous heterotrophic denitrifying microorganisms.