We studied the photoresponse of the chlorine dioxide-iodine-malonic acid (CDIMA) reaction. In a continuously fed stirred tank reactor, the photosensitivity increases with decreasing residence time and increasing concentration of chlorine dioxide. Under certain conditions, concentrations change in a sigmoidal fashion in response to a jump in the intensity of illumination. We propose a new mechanism for the effect of visible light on the CDIMA reaction. Simulations based on the proposed mechanism accurately describe the experimentally observed kinetics.