Abstract: Preparation of terephthalic acid by oxidizing p-xylene with molecular oxygen in an acetic acid solvent in the presence of a catalyst containing at least one heavy metal and bromine. Preparation of terephthalic acid of high purity accompanied by low level of combustion of the acetic acid solvent can be achieved by(i) introducing p-xylene and molecular oxygen into a first reaction zone kept at a temperature of 180.degree.-230.degree. C. to oxidize at least 95% by weight of the p-xylene into terephthalic acid;(ii) introducing the reaction mixture obtained from the first reaction zone into a second reaction zone kept at a temperature of 0.degree.-50.degree. C. below the temperature of the first reaction zone in which the reaction mixture is subjected to post-oxidation with molecular oxygen without supplying p-xylene; and(iii) introducing the reaction mixture obtained from the second reaction zone into a third reaction zone kept at a temperature of at least 230.degree. C.