Although the reduction of iodine by diamine-N-polycarboxylate ions such as ethylenediaminetetraacetic acid, trans-1,2-cyclohexanediaminetetraacetic acid, N-(2-hydroxyethyl)ethylenediamine-N,N',N'-triacetic acid, and ethylenediamine-N,N'-diacetic acid was negligible in the dark, it occurred greatly upon irradiation with visible light of an aqueous solution over the pH range 4-8. The rate of the photo-sensitized reaction was of half-order regarding both the light intensity irradiated and the concentrations of iodine, which correspond to those of the triiodide ion in the presence of excess iodide ions, and was proportional to the concentration of the diamine-N-polycarboxylate ion. Further, the rate was of half-order for concentrations of I- below 3 x 10(-4) mol dm-3 and was of inversely half-order for concentrations of I- above 6 x 10(-4) mol dm-3. The reaction was inhibited by radical scavengers of acrylonitrile, acrylamide, and hexaamminecobalt(III) ions. Both hydrogen ion and complex-forming metal ions, such as copper(II) and zinc(II), are also strong inhibitors of the reaction. The reaction mechanism, where initiation reaction is a photo-dissociation of iodine and the reactive species is a I2.- radical anion, is proposed to account for the obtained results.