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Water-soluble azo dyes are used extensively in the textile industry and are known to be problematic with respect to from textile waste waters. Under anaerobic conditions azo dyes can be utilised as terminal electron acceptors in microbial respiration, and are reduced and decolourised concurrently with re-oxidation of reduced flavin nucleotides. The microbial decolourisation of an azo dye (C.I. Reactive Red 141) was investigated with respect to the kinetic order of azo reduction and rate-controlling factors of the reaction. Decolourisation of C.I. Reactive Red 141 was found to be first order with respect to dye concentration, although increasing the initial dye concentration in the serum bottles resulted in decreasing k values of − 0.441/h (100 mg/(of C.I. Reactive Red 141), − 0.316/h (150 mg/l) and − 0.25:/h (200 mp/l). The presence of labile carbon in the anaerobic system was found to be essential in order to obtain an acceptable rate of decolourisation. The k value obtained for decolourisation of the azo dye without a supplemental carbon source (glucose) was − 0.012/h, in comparison to a k value of − 0.441/h when supplemented with glucose (1 g/l). The presence of nitrate in the anaerobic system was found to inhibit decolourisation, while the presence of sulphate was found to have to discernible effect on the rate of decolourisation. A low redox potential (− 450 to − 500 mV) was found to be conducive to rapid decolourisation of C.I. Reactive Red 141. A C.I. Reactive Red 141 depradation product was positively identified as 2-aminonaphthalene-1,5-disulphonic acid, confirming that azo reduction was responsible for decolourisation of the azo dye. A toxicity assay was performed which showed that C.I. Reactive Red 141 was inhibitory to the anaerobic microbial community at concentrations >100 mg/l, but that prior exposure of the biomass to the dye increased the resistance to previously inhibitory dye concentrations