In the present work advances and limitations in the application of Fe(III)-oxalate complexes (namely, [Fe(Ox)3]3-) to the photodegradation of a model persistent organic contaminant - 2,4-dichlorophenoxybutanoic acid (2,4-DB) in neutral aqueous solutions were systematically investigated for the first time. It has been shown that the efficiency of [Fe(Ox)3]3- system greatly depends on the initial concentrations of oxalate ion due to the fast consumption of the ligand during photodegradation process leading to the formation of photochemically less active Fe(III) species. Efficiency of Fe(Ox)3 3− system normalized to UVA absorption at the excitation wavelength is practically independent on [Fe(III)]. Thus, it is highly probable that concentrations of Fe(III) as low as < 10−5 M could be applied in water treatment procedures using reactors with very long optical path. The system also keeps high efficiency at low concentration of pollutant (<10−5 M) though this results in higher relative consumption rate of Fe(III) and oxalate ions.