A systematic approach was followed to optimise pH, temperature and the doses of FeSO4 and H2O2 for Feton's reagent (FR) to decolourize and detoxify the solutions containing of Remazol Red 120 (RR) dye and its mixture with Remazol Brillant Blue (RB) and Remazol Yellow 84 (RY) reactive dyes. The acute toxicity of each dye composition was measured using D. magna. Optimum pH for RR singly used and for the dye mixture was found 3.5 while optimum temperature was determined 50 degrees C, 40 degrees C and 30 degrees C for 100 mg/l of RR, 200 mg/l of RR and the dye mixture, respectively. More than 98% of colour and 92% of COD removal were obtained for the dye solutions. For obtaining high colour and COD removal H2O2 concentration had to be increased 3 times when RR concentration was doubled whereas FeSO4 was to be increased 2.5 times for the dye mixture. The results obtained indicate that FR can be assurely used for complete toxicity removal and obtaining high colour and COD efficiency with no toxic effluent on D. magna for the dye solutions studied. Acute toxicity test with H2O2 was useful to evaluate the complete oxidation resulting in practically no residual H2O2 (< 3 mg/l) in the solution.