In this work, the influence of toluene gas concentration on the isolation of toluene degrading microbial communities from activated sludge was studied. Toluene biodegradation at gas phase concentration of 10 g m(-3) (R1) resulted in process instability with removal efficiencies (RE) lesser than 33 %, while operation at toluene gas phase concentrations of 300 mg m(-3) (R2) and 11 mg m(-3) (R3) was stable with RE ranging from 74 to 94 %. The consortium isolated in R1 exhibited the highest tolerance toward toluene but the lowest biodegradation performance at trace level VOC concentrations. Despite R2 and R3 showed a similar sensitivity toward toluene toxicity, the microbial community from R2 supported the most efficient toluene biodegradation at trace level VOC concentrations. The Shannon-Wiener index showed an initial biodiversity decrease from 3.2 to 2.0, 1.9 and 2.7 in R1, R2 and R3, respectively. However, while R2 and R3 were able to recover their initial diversity levels by day 48, this loss in diversity was permanent in R1. These results showed that traditional inoculum isolation/acclimation techniques based on the exposure of the inoculum to high VOC concentrations, where toxicity tolerance plays a key role, may result in a poor abatement performance when the off-gas stream is diluted.