The microbial decolourization of Reactive Red 2 (RR2) dye has been studied under anaerobic conditions. Three semicontinuous bioreactors were operated with dye concentrations-R1 (control: 0 mg RR2l(-1)), R2 (100 mg RR2l(-1)) and R3 (200 mg RR2l(-1)`). The parameters monitored were, oxidation-reduction potential (ORP), methane production, colour and chemical oxygen demand (COD) removal during the feeding cycles. The oxidation-reduction potential values for the first few days were above -150 mV, which later on decreased to less than -275 mV in all the reactors. Colour removal during the first few days of operation was due to adsorption of dye on to anaerobic biomass. However, under steady state conditions, colour removal was above 76% for both the dye containing reactors and it was due to biologically mediated degradation. Methane production and chemical oxygen demand removal in the control and dye containing reactors were almost the same. Integrated analysis of the monitored parameters indicated that, the primary mechanism of colour removal was adsorption of RR2 on to anaerobic biomass and subsequent degradation. Decolourization rates were found to be first order with respect to dye concentration, although an increase in the influent dye concentration resulted in a decrease in the rate from 0.0074 (g volatile suspended solid, VSS)(-1) h(-1) (100 mg RR2l(-1)) to 0.0039 (g VSS)(-1) h(-1) (200mg RR2l(-1)). Based on total methane production no inhibition effect of dyes was observed but total methanogenic activity (TMA) results exhibited inhibition of methanogenesis. (C) 2004 Elsevier Ltd. All rights reserved.