The cultures from an ethanol-enriched anaerobic master culture reactor were dosed with 0, 50, 100, 200, 400, 800, and 1600 mg/L of a mixture of three Procion reactive dyes: Red MX-8B, Red MX-5B, and Orange MX-2R, in order to study the colour removal and the inhibitory effect of dyes on the methanogenic degradation of ethanol that was added as a cosubstrate. The reduction in colour intensity, measured at 514 nm, was found to be similar to a trial using an inactivated culture. These results indicate that removal of colour by the unacclimated anaerobic population is due to adsorption of the dyes on the biomass. The dyes caused a competitive inhibition at doses above 200 mg/L on utilization of ethanol by the anaerobic bacteria. © 2010, CAWQ.