Abstract <p>The rate constant of the reaction of 6-amino-5-hydroxy-2,3-dimethylpyrimidin-4(3<i>H</i>)-one (<b>1</b>) with peroxyl radicals was measured in a model system of radical chain oxidation of 1,4-dioxane. Oxidation of 1,4-dioxane with atmospheric oxygen was carried out at a temperature of 333 K. Oxygen uptake was monitored using a universal differential manometric setup. It was found that 6-amino-5-hydroxy-2,3-dimethylpyrimidin-4(3<i>H</i>)-one slowed down the rate of 1,4-dioxane oxidation. The effective rate constant of the interaction of the peroxyl radical of 1,4-dioxane with 6-amino-5-hydroxy-2,3-dimethylpyrimidin-4(3<i>H</i>)-one was found: <i>fk</i>₇ = (6.2 ± 0.1) × 10⁵ L mol^–1 s^–1. The stoichiometric inhibition coefficient <i>f</i> = 1.3 ± 0.1 was calculated from the slope of the dependence of the induction period length on the concentration of 6-amino-5-hydroxy-2,3-dimethylpyrimidin-4(3<i>H</i>)-one. The experimental data indicated that 6-amino-5-hydroxy-2,3-dimethylpyrimidin-4(3<i>H</i>)-one has antioxidant properties. It was assumed that the hydroxyl group –OH located in the 5-position of the ring was the site attacked by the peroxyl radical.</p>