Vanillin almost instantly reduced 1.2 to 1.5 moles of aqueous chlorine dioxide at 20 °C. or 5 °C. and any pH between 1.2 and 6.5, and a white crystalline substance with the composition of a dihydroxyvanillin, C7H5O4(OCH3), was isolated in roughly 25% yield independently of the pH. When oxidized with aqueous sodium chlorite at 20 °C. and pH 0.5, these crystals gave another crystalline substance with the composition of a dihydroxyvanillic acid, C7H5O5(OCH3). Although both these substances decomposed readily to red oils and then to brown powders free of methoxyl groups, seven well characterized derivatives were prepared. The results showed that the substances were unsaturated, monohydroxy, diketone tautomers of a dihydroxyvanillin and the corresponding dihydroxyvanillic acid, but precise structures could not be assigned. Parallel oxidations of vanillin with aqueous sodium chlorite at 20 °C. and pH 6 proceeded at a negligible rate, but near pH 5 a reaction that often seemed autocatalytic produced about 19% of 5-chlorovanillin. At pH 4 the aldehyde C7H5O4(OCH3) was isolated in 19% yield; at pH 1 this aldehyde (15%) was mixed with 7.7% of the corresponding acid C7H5O5(OCH3), but at pH 0.5 the latter alone was produced (28%). Chlorine dioxide and sodium chlorite therefore differed markedly in their oxidizing action and in the effect of pH upon it. When acting on vanillin, both oxidants also produced deep red, unstable oils with quinone-like properties and often containing chlorine.