A reversed-phase high-performance liquid chromatographic (HPLC) method is developed for the determination of the products of liquid phase oxidation of p-xylene to terephthalic acid catalyzed by cobalt naphtenate with methyl benzoate as a solvent. Analyses are performed on a 250-×4.6-mm octadecylsilica (ODS; C 18 ) bonded phase column with a 10-×3-mm guard column in 22 min. A three- component mobile phase (methanol, acetonitrile, water) is used for gradient elution, and ultraviolet detection is selectively performed at 220 and 254 nm. The internal standard method (using cumene as the internal standard) is employed. The developed method avoids filtration, extraction, and derivatization pretreatment of the samples. acetic acid by gas chromatography and high-performance liquid chromatography (HPLC), respectively. Solid products were an alyzed after esterification by gas chromatography. Very few in dications about the analytical conditions have been given; in ad dition, two different sets of equipment and a time-consuming pretreatment of the solid samples were employed. Jacobi and Baerns (4) analyzed the reaction products of p-xylene oxidation in the absence and presence of solvent (i.e., chlorobenzene and benzoic acid methylester) by HPLC. Depending upon the number of nuclei present in the products, different mixtures of eluent have been used by maintaining a constant mobile phase flow rate (i.e., 1 mL/min) at a temperature of 34°C. No infor mation about the duration of the analysis has been reported. More recently, Cao and co-workers (5,6) investigated the liquid- phase catalytic oxidation of p-xylene to p-toluic acid and to terephthalic acid using methyl benzoate as a solvent by ana lyzing the reaction products with HPLC. For the analysis of ox idation products up to terephthalic acid formation, a relatively rapid and easy-to-use technique that represents the natural upgrade of the one developed for the detection of the products up to p-toluic acid formation (5) has been employed (6). The aim of this work was to describe in detail the develop ment and the performance of this technique that allows the rapid and simultaneous analysis of p-xylene, p-tolualdehyde, 4-methylbenzyl alcohol, p-toluic acid, 4-hydroxymethylben- zoic acid, 4-carboxybenzaldehyde, terephthaldicarboxaldehyde, terephthalic acid, methyl benzoate, and cumene. The method is free of filtration, extraction, and derivatization steps.