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), [CoIIICoIIL(OCN)3]·CH3CN (2), and [CoIIICoIIL(μ-CH3COO)2](ClO4) (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 [CoIICoIIL(μ-3,5-DTBCH2−)Na]+ have been identified.