The photooxidative decolorization of C. I. Acid Orange 7 (AO7), a commercial monoazo textile dye, in the UV/H2O2 process has been investigated in a tubular continuous-flow photoreactor. The decolorization rate follows pseudo-first-order kinetic with respect to the AO7 concentration. The rate constant of the interaction of center dot OH radicals with the AO7 molecules has been estimated through the adoption of a simplified kinetic model (6.1 x 10(8) M-1 s(-1)). This model allows for predicting the concentration of AO7 in different photoreactor lengths for different flow rates and initial concentrations of H2O2 and AO7. For electrical energy cost evaluation, the figure-of-merit electrical energy per order (E-EO) was estimated from experimental and calculated data. The results show E-EO is very sensitive to the initial concentrations of H2O2 and AO(7).