Mixed complexes of cobalt(II) and nickel(II) ions with tetrahydro-1,4-thiazin-3-one (tht), thiazolidine- 2-thione (ttz) and benzoxazole-2(3H)-thione (bot), and aliphatic dicarboxylic acids, such as oxalic, malonic and succinic, are described. The newly prepared compounds are characterized on the basis of near-infrared and far-infrared spectroscopy and electronic spectra, as well as by magnetochemical investigations. All physical measurements agree with a polymeric linear-chain structure of the complexes in which the oxalate, malonate or succinate dianion acts as a tetradentate bridging ligand linking two metal ions. The metal ions attain six-coordination through the four oxygens of the anion and the donor atoms of two hetero ligands acting always as monodentate oxygen donors in the case of tht and monodentate nitrogen donors in the case of ttz and bot. The complexes having a 1 : 1 metal/ligand molar ratio achieve an octahedral structure through the coordination of a water molecule. Spectroscopic parameters for the cobalt and nickel complexes are compared with similar chromophores containing sulfur, oxygen and nitrogen donor atoms; the Dq values are in keeping with the infrared results and with literature data for MO₆, MN₂O₄ and NiNO₅ chromophores.