The Water Engineering Research Laboratory, Office of Research & Development, U.S. Environmental Protection Agency (EPA) is conducting research designed to develop techniques for predicting the fate of azo dyes in typical wastewater treatment systems which are treating wastewater contaminated with azo dyes. These techniques are being developed for EPA Office of Toxic Substance's use in the Premanufacture Notification (PMN) review process under section 5 of the Toxic Substances Control Act (TSCA) and potentially in the regulation of hazardous chemical substances and mixtures under TSCA section 6. A part of this research involves pilot plant investigations on the removal of azo dyes by the activated sludge process. Azo dyes are of concern to OTS because some of the dyes, dye precursors or their degradation products have been shown to be, or are suspected to be, carcinogenic. Therefore, the principal objectives of this research project will be to determine the fate of the dye compounds in the activated sludge treatment process. The results from this research are needed by OTS to aid in predicting the degree to which new azo dyes and/or their degradation products pass through the activated sludge process. This research will, hopefully, lead to the development of a predictive model of dye compound behavior based upon chemical structure, physical properties and/or dye classification.