The examined H2O2/Fe0 process was found to be very efficient for discoloration of simulated wastewater containing 100 mg/dm3 commercial azo dye Acid Red 18. The optimal doses of H2O2 and Fe0 were 60 and 50 mg/dm3, respectively at pH 3 and 15 minutes reaction time. Under the described conditions total visual discoloration was achieved. A very important factor was reaction time. Generally, the smaller dose of iron powder, the greater reaction time needed for visual discoloration. A strict linear correlation was observed between Fe0 amount in the range of 10–600 mg/dm3 and final pH. This suggests that the H2O2/Fe0 process has undoubted advantages in comparison with the classical Fenton reaction, because using excessive amounts of the iron powder could reach higher final pH in the H2O2/Fe0 process. Thus, smaller doses of base will be required for final neutralization of wastewater. It was also found that the dissolution time of iron powder at acidic conditions (before H2O2 was added) is an important parameter that influences the rate of discoloration. As the time of iron powder dissolution increases, the time needed for visual discoloration decreases.