Factors affecting biofilm removal of azo dyes from a synthetic, municipal-type wastewater were investigated using lab-scale, rotating drum biofilm reactors. Among the three azo dyes studied - Acid Orange 8, Acid Orange 10, and Acid Red 14 - only AO-8 degraded aerobically. The azo bond cleavage occurred very easily for all three dyes under anaerobic conditions. AO-8 removals ranged from 20% to 90%. Statistically designed experiments were used to characterize the response of pseudo-steady state biofilms. Two AO-8 removal rate maxima were identified - one at high bulk-phase dissolved oxygen and low COD removal flux, and the other at low dissolved oxygen and high COD flux. The presence of azo dyes, along with other factors such as COD loading, bulk-phase DO level and shear force, showed impact on biofilm accumulation.