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Methylene blue (MB) is a representative of a class of dyestuffs resistant to biodegradation. Its decomposition was examined in aqueous TiO₂ dispersions under UV illumination to assess the influence of temperature, pH, concentration of dissolved oxygen (DOC), initial concentration of MB, and light intensity on the kinetics of decomposition. Hypsochromic effects (i.e. blue shifts of spectral bands) resulting from N-demethylation of the dimethylamino group in MB occurs concomitantly with oxidative degradation. The maximum quantity of MB adsorbed on TiO₂, and the kinetics of degradation of MB and of total organic carbon (TOC) removal were also measured at constant pH 4. Photobleaching of MB solutions takes place at low DOCs and is caused by a reversible reductive process involving photogenerated electrons on TiO₂. The rate of degradation of MB remains fairly constant regardless of whether the dispersion …