The thermal decomposition of sodium chlorate catalyzed by manganese(IV) oxide, nickel(II) oxide and copper(II) oxide in the presence of barium peroxide or hydroxide has been studied by thermogravimetric and differential thermal analysis. Barium peroxide and hydroxide are moderately active catalysts when used alone, but both become inhibitors for the decomposition of sodium chlorate catalyzed by manganese(IV) oxide, nickel(II) oxide and copper(II) oxide. The inhibiting mechanism is discussed.