Primary effluent from a municipal wastewater treatment plant was used as the feed in bench scale activated sludge systems. These systems were spiked with disperse azo dyes at 1 mg 1-1 and 5 mg 1-1 levels and were sampled at various points in the process. Samples were analysed by high performance liquid chromatography with UV = visible detection and by thermospray ionization MS and tandem mass spectrometry (MS/MS) using direct injection or via column chromatography. The tandem mass spectrometry techniques were used both for method development purposes and for the specificity and extra information these techniques can provide. The investigation of the fate of disperse azo dyes in the activated sludge process was a major feature of this study. Major degradation products have been identified by tandem mass spectrometry analyses of these wastewaters. Precision and accuracy data generated by the thermospray tandem mass spectrometry technique are compared to those derived from the high performance liquid chromatography/UV-visible method.