The photochemical decolorization of C.I. Acid Red 27 (AR27), an anionic monoazo dye, was studied in the UV/H 2 O 2 process by using a batch reactor with a UV-C lamp emitting at 254 nm (30 W). The decolorization rate follows pseudo-first order kinetic with respect to the AR27 concentration. The results indicate that the apparent reaction rate constant in the UV/H 2 O 2 process is a function of H 2 O 2 concentration. In this work, a mathematical relation between the apparent reaction rate constant of the AR27 removal and used H 2 O 2 was established. The applied amount of H 2 O 2 was performed in two forms: (i) the light fraction absorbed by H 2 O 2 in 254 nm, (ii) the initial concentration ratio of H 2 O 2 to AR27. The results obtained from this mathematical model are in good agreement with experimental results.