The H2O2/Pyridine/Cu(II) process was found to be very effective in the decolorization of synthetic dyes. The efficiencies achieved with Cu(II) as the catalyst were over 90% after 10 min of reaction for all dyes except for Rifacion Yellow HE-4R and Levafix Yellow Brown E-3RL which were more resistant to oxidation and have decolorization efficiencies of 74% and 68.8%, respectively, after 60 min of reaction in the presence of 125 mM H2O2, 0.5% pyridine and 0.1 mM Cu(II). The rate of decolorization was not affected by pH in the range of 3-9. At pHs 2 and 11, however, a decrease in the initial rate of reaction was monitored, at pH 11, the initial rate being much more slower than that observed at pH 2. The effectiveness of the metals was in the rank of Cu(II) > Co(II) > Zn(II) > Fe(II) = Mn(II). When compared with H2O2/Pyridine system, the H2O2/pyridine/Cu(II) process was more advantageous due to the shorter reaction time required to reach a certain decolorization efficiency although the decolorization efficiencies of both processes were almost same at extended reaction periods. The decolorization efficiency increased with increasing pyridine and H2O2 concentration until a certain value after which the increase in concentrations of both had negligible effect. Further increases in Cu(II) concentration, however, led to decreases in decolorization. The decolorization of Remazol-Turquoise Blue G-133 with H2O2/pyridine system was found to follow zero order reaction rate for low concentrations and first order for high concentrations with high r(2) values. (c) 2005 Elsevier Ltd. All rights reserved.