Two compounds, [CoIII(L)(py)3][CoII(py)Cl3]EtOH and [CoIII(L)(py)3]I3(H2L = salicylaldehyde S-methylisothiosemicarbazone, py = pyridine), were synthesized and the crystal structures determined by single-crystal X-ray diffraction. In both structures the geometries of the cation are very similar to a thiosemicarbazide-based ligand coordinated in the mer configuration. The axial pyridines are in mutual perpendicular orientation, the angles between the planes being 85.3(2) and 82.5(2)°. The plane of the equatorial pyridine is tilted with respect to the equatorial plane by about 40°. The orientations of the pyridines were studied in model systems by quantum chemistry calculations. It was shown that the interactions between axial and equatorial pyridines are responsible for the orientation of pyridines in the complex cation; consequently, there are very similar geometries of the complex cation in both crystal structures. The compounds were also characterized by elemental analysis, molar conductivity, magnetic susceptibility and electronic absorption spectra. © 2001 Elsevier Science Ltd. All rights reserved.