At pH 0.5-5.0, a closed system containing an aqueous mixture of chlorine dioxide, iodine, and a species such as malonic acid (MA) or cthyl acetoacetate, which reacts with iodine to produce iodide, shows periodic changes in the light absorbance of I;. This behavior can be modeled by a simple scheme consisting of three component reactions: ( I ) the reaction between MA and iodine, which serves as a continuous source of I-; (2) the reaction between ClO,' and I-, which acts as a source of C102-:and (3) the self-inhibited reaction of chlorite and iodide that kinetically regulates the system. The fast component reaction between chlorinc dioxide and iodide ion was studied by stopped-flow spectrophotometry. The rate law is -[C102']/df =6X IO3(M-2s-')[CIO,'][I-]A.two-variablemodelobtainedfromtheempiricalratelawsofthethreecomponentreactions givcs a good dcscription of the dynamics of the system. The oscillatory behavior results not from autocatalysis but from the self-inhibitory character of the chlorite-iodide reaction.