The monomeric cobalt-phosphite-thiolato complex [Co (mpt)₂ {P (OCH₃)₃}₂ ] BF₄ (Hmpt = 2-mercaptothiazoline) has been prepared and characterized by X-ray crystallography. The complex crystallizes in the monoclinic space group C2/c with a = 0.8078(5), b = 2.6020(18), c = 1.2191(7) nm, β = 99.38 (1)°, V= 2.528(3) nm₃, and Z = 4. The structure comprises discrete cations [Co (mpt)₂{P(OCH₃)₃}₂]⁺ and anions BF₄^- , in which the cobalt (III) atom is coordinated to two chelate mpt^- and two cis-oriented monodentate P(OCH₃)₃ ligands in a highly distorted octahedral geometry. The most distorted angles are S(2) -Co(1) - S(2a) of 162.23(10)° and N(1) - Co(1)-S(2) of 71. 47 (13)°, the latter is caused by the geometric constraint of the bidentate ligand mpt^- . Cyclic voltammetry has been used to study the electrochemical behavior of the title complex on the Pt electrode in MeCN solution with 0.1 mol·L^-1 of Bu₄ⁿNBF₄ as electrolyte. The results indicate that the title complex is unstable in MeCN.