Elementary kinetic modeling was used to study the mechanism of chlorate formation in chlorine dioxide delignification. Reaction conditions reflecting typical industrial processes (T = 50°C, pH 1.5–4) were examined. Fe mediated Cl(III) decomposition and a reaction between hypochlorous acid and chlorous acid (or their equilibrium counterparts) were found to be the major reaction routes responsible for chlorate formation at pH < 3. The latter route accounts for chlorate formation at pH ≥ 3. The rate of chlorous acid (HClO2) self-decomposition was too slow either to compete against the other routes (pH < 3) or to yield notable amounts of chlorate within the given time frame (pH ≥ 3). The results suggest that chlorate formation could be suppressed, without adverse effects on chlorine dioxide regeneration, by aiming for end pH 3–3.5, ensuring a moderate chloride ion concentration and by favoring concentrated solutions/suspensions.