Fe3O4 magnetic nanoparticles (MNPs) were employed for electro-Fenton (Fe3O4-electro-Fenton) degradation of C.I. Reactive Blue 19 (RB19) in an undivided electrochemical reactor with an activated carbon fiber felt cathode and a platinum anode. On the basis of physicochemical characterization of the Fe3O4 MNPs as well as quantitative measurements of iron leaching and H2O2 generation, it is concluded that the Fe3O4 MNPs facilitated the decomposition of H2O2 to generate hydroxyl radicals (center dot OH). Moreover, the cathodic electro-Fenton facilitated electro-regeneration of ferrous ion and maintained continuous supply of H2O2. The effect of several operational parameters such as pH, current density, amount of added Fe3O4 MNPs, initial RB19 concentration, and temperature on the removal of total organic carbon was investigated. It was found that the Fe3O4-electro-Fenton degradation of RB19 followed two-stage first-order kinetics with an induction period and a rapid degradation stage. Mineralization of RB19 proceeded rapidly only at pH 3.0. Increasing the current density and the dosage of Fe3O4 MNPs enhanced the rate of RB19 degradation. However, higher current densities and Fe3O4 dosages inhibited the reaction. The rate of RB19 degradation decreased with the increase in initial RB19 concentration and increased with the increase in temperature. The removal efficiency of total organic carbon reached 87.0% after 120 mm of electrolysis at an initial pH of 3.0, current density of 3.0 mA/cm(2), 1.0 g/L concentration of added Fe3O4 MNPs, 100 mg/L initial dye concentration, and 35 degrees C temperature. On the basis of the analytical results for the intermediate products and the assumption that center dot OH radicals are the major reactive species, we propose a possible pathway of RB19 degradation during the cathodic electro-Fenton process using Fe3O4 MNPs as iron source.