The main advantage of Fenton's reagent (FR) over other (OH)-O-center dot systems is its simplicity. FR has the potential for widespread use in treating wastewater, but compared to other (OH)-O-center dot systems, little information on the dye degradation pathways of FR exists. The degradation of crystal violet (CV), a triphenyl methane dye, by FIR was determined as a function of reagent concentration and ratio and pH in the batch treatment. The experimental results showed the optimum Fe2+/H2O2 ratio to be 0.5mM:50 mM and the optimum Fe3+/H2O2 ratio to be 1 mM:50mM. Optimal pH was about 3. To obtain a better understanding of the mechanistic details of Fenton reagent's degradation of CV dye, the intermediates of the process were separated, identified, and characterized by HPLC-PDA-ESI-MS and GC-MS techniques in this study. Indications were that the probable degradation pathways were N-de-methylation and cleavage of the conjugated chromophore structure. The intermediates were generated in the order of the reaction time and relative concentration, indicating that the N-de-methylation degradation of CV dye is a major reaction pathway. The reaction mechanisms proposed in this research should prove useful for future application of the technology to the decolorization of dyes. (C) 2009 Elsevier B.V. All rights reserved.