Mixed ligand octahedral cobalt(III) complexes with the tridentate salicylaldehyde semi-, thiosemi- and isothiosemicarbazone and pyridine of general formula [CoIII(L11-3)(py)3]X (H2L1 = salicylaldehyde semicarbazone, X = [CoIICl3(py)]-, ClO4- . H2O, I- . 0.5 I2; H2L2 = salicylaldehyde thiosemicarbazone, X = [CoIICl3(py)]-, [CoIIBr3(py)]-, ClO4- . H2O, I3-; H2L3 = salicylaldehyde S-methylisothiosemicarbazone, X = [ CoIIBr3(py) ]-, ClO4- . H2O, BF4-) were synthesized. The tridentate coordination of all the three dianionic forms of the ligands involves the phenol oxygen, hydrazine nitrogen and the chalcogen (O or S) in case of salicylaldehyde semi-, thiosemicarbazone and the terminal nitrogen atom in the case of isothiosemicarbazone. For all the complexes, a meridial octahedral arrangement is proposed, which is a consequence of the planarity of the chelate ligand. The compounds were characterized by elemental analysis, molar conductivity, magnetic susceptibility, IR and electronic absorption spectra. The thermal decomposition of the complexes was investigated by thermogravimetry, coupled TG-MS measurements and DSC.