Azo dyes are common contaminants in wastewater. Many are poorly removed by most typical municipal treatment processes. Those which are partially degraded may form toxic intermediates, particularly under anaerobic conditions. Acid Orange 7 (AO-7) is a simple azo dye which is biotransformable. In this study, bulk-phase factors affecting azo bond cleavage of AO-7 in a synthetic municipal wastewater were investigated using lab-scale, rotating drum biofilm reactors. A series of statistically designed experiments were used to characterize the response of the pseudo-steady state biofilms. A variety of microorganisms from the activated sludge seed for the biofilm were found to be capable of transforming the AO-7. Biofilm removals of AO-7 ranged from 18 to 97%. Two maxima of AO-7 transformation rates were found-one at high bulk-phase dissolved oxygen and low COD removal flux, and another at low dissolved oxygen and high COD flux. No 1-amino 2-naphthol intermediate was detected. The sulfanilic acid intermediate was present at low dissolved oxygen levels. Suspended-phase COD removal was inhibited by AO-7, but the effect was not detected in the biofilm reactor system. AO-7 transformation and biological nitrification interact, but the impact is small.