We show that illumination of the chlorine dioxide-iodine-malonic acid reaction with visible light suppresses oscillations and shifts the steady state of the reaction to lower concentrations of iodide ions. In the system with starch, illumination results in a strong decrease of the steady-state concentration of the triiodide-starch complex. We suggest a simple mechanism, in which iodine atoms produced by photodissociation of molecular iodine initiate reduction of chlorine dioxide to chlorite and oxidation of iodide ions to iodine. This results in a decreased amplitude of oscillations and, at more intense illumination, the cessation of oscillations. Illumination also lowers the steady-state concentrations of iodide and the triiodide-starch complex. Results obtained from numerical simulations are in good agreement with the experimental data.