The present work describes the use of ozone to degrade selected reactive dyes from the textile industry and the analysis of the resulting complex mixture by liquid chromatography/mass spectrometry (LC-MS). To allow certain identification of the substances detected in the wastewater, the original dyes were also investigated either separately or in a synthetic mixture of three dyes (trichromie). Since the reactive dyes are hydrolyzed during the dyeing process, procedures for the hydrolysis were worked out first for the individual dyes. The ozonated solutions were concentrated by solid-phase extraction, which separated very polar or ionic substances from moderately polar degradation products. The latter, which are the primary degradation products, were investigated by liquid chromatography/mass spectrometry with a tandem quadrupole time-of-flight mass analyzer. Accurate masses, which in most cases could be determined with a deviation of <or=5 ppm from the exact value, were also measured. In addition, a diode-array detector was placed before the mass analyzer to provide UV-vis spectra of the products in the same run. With retention times, mass spectra, accurate masses, UV-vis spectra and, of course, knowledge of the structures of the original dyes, plausible structures could be proposed for most of the components of the moderately polar fraction. These structures were confirmed by 1H NMR in cases where it was practical to isolate the degradation products by preparative HPLC.