Oxidation of water on an illuminated TiO2 electrode was investigated using a rotating ring-disk electrode, focusing on the role of the reducing agent, SO32-, added in solution. The TiO2 disk was illuminated with a chopped-light source, and the corresponding ring response at the Pt-ring, Delta I-R = I-R (light) - I-R (dark), was recorded. Although oxidation products were expected to be produced on the disk surface and carried to the ring electrode, Delta I-R was found to be negative in dilute Na2SO3 solution even at large negative potentials, e.g., -0.8 V vs. SCE. This phenomenon was observed in neutral and basic solutions. It is proposed that SO3-* radical is formed at an illuminated TiO2-disk and subsequently initiates a homogeneous free-radical chain oxidation of sulfite ion. This chain reaction consumes oxygen to be supplied from the solution via the disk to the ring, reducing the ring current associated with the reduction of oxygen. As a result, the ring current is lower under illumination than in the dark.