The autoxidation of p-xylene (pX) was carried out with the Co(OAc)(2)/Mn(OAc)(2)/Br- catalyst in the presence of a strong acid, such as trifluoroacetic acid (TFA), heptafluorobutyric acid (HFBA) and p-toluenesulfonic acid. The initial reaction rates, measured by the rate of oxygen uptake, and the yields of terephthalic acid were used to compare the effectiveness of acid additives. All the acid additives improved significantly the effectiveness of the catalyst at their low concentrations. The autoxidation reaction increased with the Mn(OAc)(2) concentration. The reaction involves the formation of manganese(III) salts of strong acids, which are more powerful oxidizing agents than manganese(Ill) acetate. Thus, the cycles of free-radical chain reactions that involve formation of dibromide radical take place rapidly. Dibromide radical abstracts a hydrogen atom from the methyl group of p-xylene to form ArCH2center dot, and then the sequence progresses through alcohol, aldehyde and carboxylic acid. Manganese(III) salts of strong acids also directly react with p-xylene. This was confirmed by studying the reaction between Mn(OAc)3 and p-xylene in the presence of trifluoroacetic acid. (c) 2005 Elsevier B.V. All rights reserved.