Fe(III) polycarboxylate complexes in photo-Phenton systems are investigated as perspective agents in advanced oxidation processes (AOPs). They demonstrate high photolysis‘ quantum yields and great efficiency in •OH generation under UV excitation. Hydroxyl radicals are able to non-selective oxidation of large number of organic pollutants. As ligands in those complexes can be used widely known ethylendiaminetetraacetic acid (EDTA) or its structure isomer – etilendiamindisuccinic acid (EDDS). Advantage of EDDS, in contrast of EDTA, consists in the fact, that EDDS is naturally recycled by some types of bacteria. However, there is very little information about photolysis‘ mechanism and the quantum yields of •OH radicals (OH) upon excitation of this system. This work presents a detailed study of complex photolysis‘ efficiency and its efficiency in •OH radicals‘ generation in a wide range of experimental conditions (pH, excitation wavelength, oxygen content, initial ligand and Fe(III) concentrations). Using steady-state and laser flash photolysis methods in combination with spectrophotometry and high performance liquid chromatography, we got the following results: 1. Primary intermediate is a long-lived (1.8 ms) radical complex [Fe(II)…EDDS•], which is formed in the electron transfer from ligand to Fe(III) ion (max = 520 nm). 2. Quantum yields were showed to be dependent from initial concentration of iron, ligand, dissolved oxygen and solution acidity and have achieved value ~0.7 in optimal conditions at neutral pH. 3. •OH quantum yield was showed to be dependent from initial concentration of iron, ligand and solution acidity and has achieved value ~0.1 in optimal conditions at neutral pH. Received results are important in understanding both fundamental photochemistry of Fe(III) coordination compounds with polydentate carbonic acids and for application of such complexes in advanced oxidation processes of wastewater treatment. Asknowledgement: This work was financially supported by the Russian Science Foundation (No. 21-43-00004).