In this study, online spectrophotometric method was used to follow the decolorization of Acid Orange 8 (AO8) and Acid Red 44 (AR44) dyes from aqueous solution by Fenton process. This method allowed both the in situ monitoring of spectral changes and the determination of dye concentration during the oxidation process. The effects of initial pH value, H2O2, Fe2+, dye, and salt concentrations and reaction temperature on the decolorization of azo dyes were investigated. It was found that the optimum concentration ratios of H2O2:Fe2+ for AO8 and AR44 were 30:1 and 15:1, respectively. In addition, the highest decolorization amount for dyes was obtained at pH 3.5. The addition of NaCl into dye solution led to a negative effect on the decolorization of azo dyes; however, Na2SO4 did not affect the dye decolorization. The destruction of azo groups in AO8 and AR44 dyes occurred quickly while the destruction of benzene and naphthalene rings in these dyes progressed slowly. The kinetic data were treated according to the first-order, the second-order, and Behnajady-Modirshahla-Ghanbary kinetic models and the best model representing the experimental kinetic data was found to be Behnajady-Modirshahla-Ghanbary kinetic model. This study showed that online spectrophotometric method is very practical and easy. It can be used to follow the removal of pollutants from waters. Moreover, many experimental data can be obtained with this method.