Reaction paths governing chlorine dioxide formation from solutions of sodium chlorate, sulfuric acid, hydrogen peroxide and sodium chloride at 25°C were examined. Chlorine dioxide was formed by two paths: the reduction of chlorate by hydrogen peroxide and the reduction of chlorate by chloride. At conditions of this study, the rate of the chloride-chlorate path predominated and was enhanced by hydrogen peroxide, reaching an upper limit as hydrogen peroxide concentration was increased. A mechanism was proposed that explains the observed behavior. It is based on existing theory, but includes a new step: the rapid reaction of hydrogen peroxide with an intermediate, Cl2O2. It was shown also that chloride was not required to sustain chlorine dioxide formation in the hydrogen peroxide-chlorate path.