Four new organotin(IV) complexes of bis-(2,6-di-tert-butylphenol)tin(IV) dichloride [(tert-Bu-)(2)(HO-Ph)](2)SnCl(2) (1) with the heterocyclic thioamides 2-mercapto-pyrimidine (PMTH), 2-mercapto-4-methyl-pyrimidine (MPMTH), 2-mercapto-pyridine (PYTH) and 2-mercapto-benzothiazole (MBZTH), of formulae {[(tert-Bu-)(2)(HO-Ph)](2)Sn(PMT)(2)} (2), {[(tert-Bu-)(2)(HO-Ph)](2)Sn(MPMT)(2)} (3), {[(tert-Bu-)(2)(HO-Ph)](2)SnCl(PYT)} (4) and {[(tert-Bu-)(2)(HO-Ph)](2)SnCl(MBZT)} (5), have been synthesized and characterized by elemental analysis, (1)H-, (13)C-, (119)Sn-NMR, EPR, FT-IR, Raman and Mössbauer spectroscopic techniques. The crystal and molecular structures of compounds 1–5 have been determined by X-ray diffraction. The geometries around the metal center adopted in complexes 1–5 varied between tetrahedral in 1, trigonal bipyramidal in 3, 4, 5 and distorted octahedral in 2. Two carbon atoms from aryl groups and two chlorine atoms form a distorted tetrahedron in the case of 1. Two carbon, two sulfur and two nitrogen atoms from thione ligands form a distorted octahedral geometry around tin(IV) with trans-C(2), cis-N(2), cis-S(2)-configurations in 2. However, in the case of 4 and 5 complexes two carbon, one sulfur, one nitrogen and one chloride atom form a distorted trigonal bipyramidal arrangement. Finally, in the case of 3 the trigonal bipyramidal geometry is achieved by two carbon, two sulfur and one nitrogen atom in a unique coordination mode of thioamides toward the tin(IV) cation. Compounds 1–5 were tested for their in vitro cytotoxicity against the human breast adenocarcinoma (MCF-7) cell line. Compound 3 exhibits strong cytotoxic activity against MCF-7 cells (IC(50) = 0.58 ± 0.1 μM).