Decolorisation of azo dyes during biological effluent treatment can involve both adsorption to cell biomass and degradation by azo-bond reduction during anaerobic digestion. Degradation is expected to form aromatic amines, which may be toxic and recalcitrant to anaerobic treatment but degradable aerobically. Methods for the quantitative detection of substituted aromatic amines arising from azo-dye cleavage are complex. A simple qualitative method is suggested as a way in which to investigate whether decolorisation is actually due to degradation, and whether the amines generated are successfully removed by aerobic treatment. Samples from a combined anaerobic-aerobic system used for treating a simulated textile wastewater containing the reactive azo dye Procion Red H-E7B were analysed by high-performance liquid chromatoraphy/ultraviolet (HPLC-UV) methods. Anaerobic treatment gave significant decolorisation, and respiration-inhibition tests showed that the anaerobic effluent had an increased toxicity, suggesting azo-dye degradation. The HPLC method showed that more polar, UV-absorbing compounds had been generated. Aerobically, these compounds were removed or converted to highly polar compounds, as shown by HPLC analysis. Since the total organic nitrogen (TON) decreased aerobically as organic N-containing compounds were mineralised, aromatic amine degradation is suggested. Although only a simple qualitative HPLC method was used, colour removal, toxicity and TON removal all support its usefulness in analysing biotreatment of azo dyes.