5-Hydroxyisophthalic acid is of practical interest in the synthesis of polymers, drugs, cosmetics. The methods for its preparation described in the literature are characterized by a number of significant drawbacks, which hinders the use of this compound in the development of new materials with valuable consumer properties. In this regard, a method for the synthesis of 5-hydroxyisophthalic acid by the oxidation of 3, 5-xylenol has been developed. It has been established that the hydroxyl group inhibits the oxidation of methyl groups; therefore, to carry out oxidative transformations, the hydroxyl group was replaced by acetyl. For this, 3,5-xylenol was acylated with acetic anhydride in the presence of sulfuric acid. The resulting 3, 5-dimethylphenylacetate is oxidized by atmospheric oxygen in an acetic acid medium in the presence of a catalytic system containing cobalt acetals manganese and bromide ions up to 5-acetoxyisophthalic acid. The effect of temperature on the reaction was studied and the activation energy was determined. It was 50 ± 5 kJ/mol that corresponds to the activation energy of oxidation of alkylaromatic hydrocarbons. The dependence of the rate of the process and the yield of the target product on the concentrations of the starting compound and the components of the catalytic system is investigated. It has been established that carrying out the oxidation process of 3,5-dimethylphenylacetate at its initial concentration of 0.75 mol/l, temperature 110 °C and cobalt (II) acetate concentration 0.1 mol/l, sodium bromide 0.05 mol/l, manganese (II) acetate 0.003 mol/l provides the reaction with high rate and the yield of 5-acetoxyisophthalic acid 70-75%. In these conditions, the balance experiment was carried out. 5-Acetoxyisophthalic acid is of practical interest as a monomer for heat-resistant polymers. To obtain 5-hydroxyisophthalic acid, 5-acetoxyisophthalic acid is subjected to alkaline hydrolysis.