Abstract A novel cobalt(III) complex with d- and/or l-selenomethioninates (d- and/or l-semet) functioning as terdentate-N,O,Se ligand was prepared and separated into the five geometrical isomers, trans(N), trans(O), trans(Se), cis,cis,cis, and trans,trans,trans. Of these isomers trans(N)- and trans(O)-[Co(d- or l-semet)2]+ and cis,cis,cis-[Co(d-semet)(l-semet)]+, which were obtained as the crystalline state, were optically resolved by the column chromatographic method. All the isomers were characterized by the electronic absorption, 1H NMR, and CD spectra. The trans(N), trans(O), and cis,cis,cis isomers in the crystalline state take the selective configurations with respect to the absolute configuration, R(Se) or S(Se), of the two coordinated selenium atoms. While, these isomers in an aqueous solution are a mixture of some configurations in the equilibrium state. For the inversion on the asymmetric selenium donor atom of the trans(N) and cis,cis,cis isomers, the rate constants and the ΔG\eweq values are investigated on the basis of the absorption and 1H NMR spectral changes. The trans(N) and cis,cis,cis isomers showed the characteristic CD spectral behavior in the d–d absorption band region depending on the inversion on the asymmetric selenium donor atoms.