Abstract Photo-Fenton processes activated by biodegradable Fe(III)–EDDS complexes have attracted huge attention from the scientific community, but the operative mechanism of the photo-activation of H 2 O 2 in the presence of Fe(III)–EDDS has not been fully clarified yet. The application of the Fe(III)–EDDS complex in Fenton and photo-Fenton (mainly under UV-B light) processes, using 4-chlorophenol (4-CP) as a model pollutant was explored to give insights into the operative mechanism. Furthermore, the potential synergistic contribution of soybean peroxidase (SBP) was investigated, since it has been reported that upon irradiation of Fe(III)–EDDS the production of H 2 O 2 can occur. SBP did not boost the 4-CP degradation, suggesting that the possibly produced H 2 O 2 reacts immediately with the Fe(II) ion with a quick kinetics that does not allow the diffusion of H 2 O 2 into the bulk of the solution (i.e., outside the solvent cage of the complex). So, a concerted mechanism in which the photochemically produced H 2 O 2 and Fe(II) react inside the hydration sphere of the Fe(III)–EDDS complex is proposed. Graphical abstract