Water solutions of direct dyes such as Ostazine Yellow H-5G (C.I. Reactive Yellow 3), Ostazine Blue H-P (C.I. Reactive Blue 13), and Ostazine Green H-3G (C.I. Reactive Green 8) were decolourized by the Fenton reaction using zero-valent iron (Fe shavings) as a source of Fe²⁺ cations. The best results were obtained using 0.05 ml of conc. sulfuric acid, reaction time 2 h, and successive addition of hydrogen peroxide during the reaction. In all experiments the colour was completely removed. The used method also led to a significant decrease in chemical oxygen demand in waste waters.