This research aimed to evaluate the effect of nitrate on anaerobic azo dye reduction by using mesophilic bioreactors, in the absence (reactor R2) and in the presence (reactor R1) of redox mediators. The azo dye Reactive Red 2 (RR2) and the redox mediator anthraquinone-2,6-disulphonate (AQDS) were selected as model compounds. The results showed that the bioreactors were efficient on RR2 reduction, in which ethanol showed to be a good electron donor to sustain dye reduction under anaerobic conditions. The redox mediator AQDS increased the rates of reductive decolourisation, but its effect was not so remarkable compared to the previous experiments conducted. Contrary to the raised hypothesis that the nitrate addition could decrease decolourisation rates and catalytic properties of the redox mediators, no effect of nitrate was observed in the bioreactors, suggesting that the presence of nitrate in textile wastewaters will not decrease the capacity of anaerobic reactors supplemented or not with redox mediators to decolourize azo dyes.