with the same ratios during the treatment and faster than the COD (Fig. 6, inset). This fact indicates that, in agreement with the mechanism proposed by Liu et al. (Liu et al., 1999), the first step in the oxidation of alizarin red is the cleavage of the aromatic ring in the C–C bond near the C]O group to form colourless intermediates (mainly phthalic acid, small carbonyl species) and finally these intermediates are mineralised to carbon dioxide.