The main advantage of Fenton's reagent (FR) over other OH systems is its simplicity. FR has the potential for widespread use in treating wastewater, but compared to other OH systems, little information on the dye degradation pathways of FR exists. The degradation of crystal violet (CV), a triphenylmethane dye, by FR was determined as a function of reagent concentration and ratio and pH in the batch treatment. The experimental results showed the optimum Fe(2+)/H(2)O(2) ratio to be 0.5mM:50mM and the optimum Fe(3+)/H(2)O(2) ratio to be 1mM: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.