The work in this paper presents the syntheses, characterization, catecholase activity, and electrospray ionization mass spectroscopic (ESI-MS positive) study of three mixed-valence dinuclear CoIIICoII complexes of composition [CoIIICoIIL(N3) 3]·CH3CN (1), [CoIIICo IIL(OCN)3]·CH3CN (2), and [Co IIICoIIL(μ-CH3COO)2](ClO 4) (3), derived from a tetraimino diphenolate macrocyclic ligand H2L, obtained on [2 + 2] condensation of 4-ethyl-2,6-diformylphenol and 2,2′-dimethyl-1,3-diaminopropane. While 1 and 2 are diphenoxo-bridged, 3 is a heterobridged bis(μ-phenoxo)bis(μ-acetate) system. Utilizing 3,5-di-tert-butyl catechol (3,5-DTBCH2) as the substrate, the catecholase activity of all the three complexes has been checked in methanol/acetonitrile/N,N-dimethyl formamide. While 2 and 3 are inactive, complex 1 shows catecholase activity with turnover numbers of 482.16 h -1 and 45.38 h-1 in acetonitrile and methanol, respectively. Electrospray ionization mass (ESI-MS positive) spectra of complexes 1-3 have been recorded in acetonitrile solutions and the positive ions have been well characterized. The ESI-MS positive spectrum of complex 1 in the presence of 3,5-DTBCH2 has also been recorded and, interestingly, two positive ions [CoIIICoIIL(N3) 2(3,5-DTBCH-)H]+ and [CoIICo IIL(μ-3,5-DTBCH2-)Na]+ have been identified. © The Royal Society of Chemistry 2013.