This study was designed to decolorize and to reduce COD content in a mixture of four reactive dyes, i.e., Remazol Black 5 (RB5), Remazol Red RB (RR), Remazol Yellow 84 (RY), Remazol Brilliant Blue (RB) using Fenton Oxidation Process (FOP). Optimum pH, temperature, and the doses of FeSO4 and H2O2 were determined. Experiments were conducted on the samples containing a total concentration of 100 mg L-1 (RB+RY), 200 mg L-1 (RB5+RR), 300 mg L-1 (RB5+RR+RB+RY), and 400 mg L-1 (RR+RB+RY) dyes considering their actual application doses in dyehouses. Optimum pH was observed as 2.5 at 30degreesC using 400 mg L-1 FeSO4 and 800 mg L-1 of H2O2 resulting in more than 96% COD and 99% Pt-Co unit of color removal for the mixture of RB5 and RR. The optimum conditions determined were 4.0 pH, 50degreesC, and 500 mg L-1 FeSO4 applying 1000 mg L-1 H2O2 for the mixture of (RB5+RR+RB+RY). A 100 mg L-1 solution of a mixture of RB and RY at equal amounts was oxidized using 200 mg L-1 FeSO4 and 300 mg L-1 H2O2 at 3.0 pH and 50degreesC.