Photoinduced reaction between ClO2 and I-2 has been discovered under illumination with 460 nm lightband. The photochemical reaction has a variable stoichiometry in acidic aqueous solution because the induced disproportionation of ClO2 to ClO3- and Cl- competes with the oxidation of I-2 to IO3- by ClO2 in the illuminated reaction mixture. The reaction rate depends on the light power of illumination and on the concentration of 12, but it is independent of the concentration of ClO2. It is also independent of the pH in the range of 0-2.0 and of the ionic strength in the range of 0.01-1.0 M. Reversible dissociation of 12 has been identified as the primary photochemical process and rate-determining step in the mechanism. Reactive I atoms are considered to initiate fast reaction steps, leading to the formation of products through reactive intermediates such as IClO2, ClO, IO, and HOCL. This mechanism is proposed for explaining the photoresponses of the CDIMA oscillatory reaction system to the illumination with visible light.