<p>The effects of applied electric potential (<i>E</i>) on electrochemical hydroxyl radical (·OH radical) generation were systematically investigated in an electro-Fenton system with a sacrificial iron anode and a platinum cathode. The mechanism of ·OH radical generation shifted within the applied electric potential range of <i>E</i> = − 0.5 to − 0.6&#xa0;V (vs. SHE). We elucidated that ·OH radical generation mechanisms were highly dependent on <i>E</i>. At <i>E</i> ≥ − 0.5&#xa0;V, the process was governed by the Fenton reaction between in situ generated H₂O₂ and electro-eluted Fe²⁺. At more negative potentials (<i>E</i> ≤ − 0.6&#xa0;V), sacrificial anode oxidation was hindered, shifting the mechanism towards the electrolysis of H₂O₂ and H₂O at the electrode surfaces. The behaviors of Fe ions and H₂O₂ confirmed a shift in the ·OH radical generation mechanisms between the Fenton reaction and electrolysis, depending on the applied electric potentials. The kinetics of electro-Fenton Orange II degradation, characterized by an initial induction period followed by a rapid degradation phase, was accurately modeled by a modified logistic kinetic model. Analysis of the dynamic concentration profiles of H₂O₂ and Fe ions in an electro-Fenton process with initial H₂O₂ addition revealed insight into the contributions of the Fenton reaction and electrolysis to ·OH radical generation, which varied with the applied electric potential.</p> Graphical abstract <p></p>