The behavior of water solutions of dyes under the influence of oxidizing agents has been described. Decomposition of the dyes in time was observed on the basis of changes in the adsorption spectra. The obtained spectrophotometrical data were subjected to the factor analysis in order to determine the number and the spectral characteristics of colour components in the systems under study. The correctness of the accepted mechanism of decolouration (parallel and successive reactions) was tested and the number of reaction stages was determined. The rate constants of substrate decomposition and product formation were determined. Moreover, the spectral profiles of intermediate products were obtained.