Under anaerobic conditions, Shewanella cinicaD14(T), Shewanella baltica and Shewane-lla putrefaciens are capable of high-rate azoreduction and humus reduction. The results indicated that at low concentration ( <2 mmol/L) AQS was a accelerator for bacterial azoreduction. However, when the concentration of AQS was more than 5 mmol/L a strong inhibition was occurred. On the other hand, the concentration of AQDS as high as 12 mmol/L the inhibition of azoreduction was still not exhibited, but the effect of acceleration was gradually decreased with the concentration of AQDS increasing (1 mmol/L to 12 mmol/L). 6 azo dyes (Table 1) were tested, all of which had a similar results. These results indicated that the humic substances were not only as a redox mediator during the azoreduction, but also as the competitor for electron from respiration chain. Because the humic substances could be act as the terminal electron acceptor for bacterial anaerobic respiration. AQS and AQDS exhibiting different behaviour on the azoreduction were determined by their standard redox potentials.