Novel, solid catalysts of mu (3)-oxo-bridged Co/Mn cluster complexes, viz., [Co-3(O)(CH3COO)(6)(pyridine)(3)](+), [Mn-3(O) (CH3COO)(()pyridine)(3)](+), and CoMn2(O)(CH3COO)(6)(pyridine)(3) (denoted CO3(O), Mn-3(O), and CoMn2(O), respectively), encapsulated in zeolite-Y oxidize selectively, para-xylene to terephthalic acid with dioxygen. The catalysts were prepared by the "flexible ligand synthesis" method and characterized by X-ray diffraction, thermal analysis, cyclic voltammetry, Fourier transform infrared, diffuse reflectance ultraviolet-visible, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectroscopic techniques. The various physicochemical measurements confirm the presence and structural integrity of the mu (3)-oxo-bridged cluster complexes in zeolite cavities. The activity and selectivity of both the "neat" and encapsulated cluster complexes followed the order CoMn2(O) > Mn-3(O) > Co-3(O), revealing the superiority of the heteronuclear complexes. Under optimal conditions, both neat and encapsulated cluster catalysts exhibit 100% para-xylene conversion with >98% selectivity for terephthatic acid. It is important that the key impurity, 4-carboxybenzaldehyde, is significantly lower in abundance (than the current commercial catalysts) with one of the zeolite-encapsulated catalysts, CoMn2(O)-Y. Leaching of metal ions from the solid catalyst during reaction is minimal and the catalyst could be recycled without significant loss of activity. A more facile redox behavior of Co between +2 and +3 oxidation states in CoMn2(O) (confirmed by cyclic voltammetry) is perhaps responsible for high catalytic activity.