Photocatalytic decolorization of Reactive Red 239 (RR239, a textile monoazo dye, was investigated in aqueous suspensions containing ZnO as a photocatalyst. The reaction can be mathematically described as a function of several parameters, including UV light intensity, initial pH, amount of photocatalyst, and stirring speed, using Design of Experiments (DOE). The optimal values for these four parameters were determined as follows: UV light intensity: 15.1 W/m(2); ZnO concentration: 4.34 g/L; initial pH: 2.6; and stirred speed: 855 rpm. Under this condition, the maximal decolorization efficiency of 99.98% can be achieved. Furthermore, photocatalytic decolorization of diazo (Reactive Yellow 84 (RY84)), triazo (Direct Black 166 (DBK166)), and polyazo dyes (Direct Black 22 (DBK22)) were also investigated under the optimized condition of RR239. The experimental results show that the UV/ZnO photocatalytic process exhibits slightly greater decolorization efficiency with RR239 than the other dyes.