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 Co^IIICo^II complexes of composition [Co^IIICo^IIL(N₃)₃]·CH₃CN (1), [Co^IIICo^IIL(OCN)₃]·CH₃CN (2), and [Co^IIICo^IIL(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 [Co^IIICo^IIL(N₃)₂(3,5-DTBCH⁻)H]⁺ and [Co^IICo^IIL(μ-3,5-DTBCH²⁻)Na]⁺ have been identified.