The photoinitiated and cerium(III)-catalyzed aqueous reaction between sulfite ion and oxygen has been studied in a diode-array spectrophotometer using the same light beam for excitation and detection. Cerlum(III) is identified as the photoactive absorbing species, and the production of cerium(IV) initiates a radical chain reaction. To interpret all the experimental findings, a simple scheme is proposed, in which the additional chain carriers are sulfite ion radical (SO3(-center dot)), sulfate ion radical (S04(-center dot)), and peroxomonosufate ion radical (SO5-center dot). The overall rate of oxidation is proportional to the square root of the light intensity per unit volume, which is readily interpreted by the second-order termination reaction of the proposed scheme. It is also shown that the reaction proceeds for an extended period of time in the dark following illumination, and a quantitative analysis is presented for this phase as well. The postulated model predicts that cerium(III) should have a cocatalytic or synergistic effect on the autoxidation of sulfite ion in the presence of other catalysts. This prediction was confirmed in the iron(III)-sulfite ion-oxygen system. The experimental method and the mathematical treatment used might be applicable to a wide range of photoinduced chain reactions.