The syntheses of several Co(III) complexes containing crown thioether ligands are reported. The complexes are characterized using UV-Vis absorption spectroscopy, carbon-13 and cobalt-59 NMR spectroscopy, and cyclic voltammetry. The structure of the cobalt(III) complex of the 20-membered macrocyclic hexathioether, 20S6, 1,4,7,11,14,17-hexathiacycloeicosane, has also been determined by single-crystal X-ray diffraction. The stereoisomer obtained is a racemic set in which the two trimethylene bridges are cis to each other and the two diethylene units lie in a facial relationship. Crystal data for [Co(20S6)](BF 4 ) 3 2H 2 O: C 1 4 H 3 2 B 3 CoF 1 2 O 2 S 6 ; monoclinic, space group C2c; a = 17.186(5), b = 16.014(3), c = 10.283(2), β = 98.57(2)°, V = 2798.45 9 3 ,Z = 4. The crown thioethers discussed in this paper all function as strong field ligands towards Co(III), in contrast to their behavior towards Cr(III) and V(III). Solution carbon-13 NMR studies show the presence of a single meso diastereomer for [Co(18S6)] 3 + and a single diastereomer for [Co(20S6)] 3 + . Chemical shifts for 5 9 Co NMR spectra correlate with the ligand field strengths and also show the presence of a single diastereomer for both hexathioether complexes. The electrochemical behavior of several hexakis(thioether) complexes of Co(III) is described. The synthesis of the heteroleptic complex, [Co(9S3)Cl 3 ] is reported, and this complex can be used as an intermediate in the preparation of mixed crown hexakis (thioether) complexes such as [Co(9S3)(10S3)] 3 + .