The mono- and multinuclear metal complexes present during the aerial oxidation of p-xylene by the homogeneous catalyst systems viz. Co/Br, Mn/Br, Co/Mn/Br, Co/Ce/Br, Co/Zr/Br, Co/Mn/Zr/Br, Co/Mn/Ce/Br, Ni/Mn/Br and Ni/Mn/Zr/Br in acetic acid solvent have been investigated by electronic and EPR spectroscopies. The reaction mixtures contain, in addition to Co(OAc)(2). 4H(2)O and Mn(OAc)(2). H2O, species like Co(OAc)Br, Co(OAc)(3), Co-3(O)(OAc)(x), Mn-3(O)(OAc)(x) and hetero-multinuclear complexes like Co2Mn(O)(OAc)(x) and CoMn2(O)(OAc)(x). While mononuclear Co(OAc);?, Mn(OAc), and Co(OAc)Br complexes predominate in the initial stages of the oxidation reaction, significant concentrations of multinuclear Co(III) and Mn(III) complexes are detected in the later stages. Zr(IV), when present, facilitates the oxidation of Mn(II) to Mn(III), a crucial step in the kinetic pathway of the oxidation reaction. Our EPR results indicate the: presence of home-nuclear Mn-3(O)(OAc)(x) type clusters as the major species with a minor component of CoMn2(O)(OAc)(x) complex in catalyst systems with Co:Mn = 1:3(mol). When an excess of cobalt catalyst is used (e.g. Co:Mn = 3:1) or when Zr and Br- are also present in optimal concentrations, the heteronuclear cluster Co2Mn(O)(OAc)(x) predominates while Co-3(O)(OAc)(x) and CoMn2(O)(OAc)(x) occur as minor constituents. The yield of terephthalic acid (TA) is enhanced at high concentrations of cluster complexes like Co2Mn(O)(OAc)(x) and CoMn2(O)(OAc)(x). The combination Co/Mn/Zr (3:1:0.1mol), exhibits a high catalytic activity and I;electivity for terephthalic acid, especially in the presence of an optimal concentration of the bromide ion.