Water solutions of some dyes produced by Bayer and Sandoz and waste waters from three plants were decolourized with Fenton's reagent. The reduction in chemical oxygen demand ranged from 60 to 89% and from 24 to 94% for individual dyes and waste waters, respectively. The mean decolourization of dye solutions was above 98% and that of waste waters from Duslo, Sala and Merina, Trencin was 100%. The decolourization of pulp-mill waste waters (Sturovo) was inefficient. The found efficient conditions of the treatment were: pH 3.0, H₂O₂-to-Fe₂SO₄ ratio 1.75:(1.00-2.00), Fenton reaction time at 24^oC 1 hour, pH for coprecipitation 7.0. Better results are obtained by addition of a flocculant during neutralization. Fenton's reagent is efficient in decolourizing waters that also contain dissolved organics.