Reactions of (H(2)azole)(2)[OSCl6], where Hazole = pyrazole, Hpz, (1), indazole, Hind, (2), imidazole, Him, (3) and benzimidazole, Hbzim, (4) with the corresponding azole heterocycle in 1:4 molar ratio in boiling isoamyl alcohol or hexanol-1 afforded novel water-soluble osmium(111) complexes of the type trans-[OsCl2(Hazole)(4)]Cl, where Hazole = Hpz (5a), Hind (6a), Him (7a), and Hbzim (9a) in 50-70% (5a, 7a, 9a) and 5% (6a) yields. The synthesis of 7a was accompanied by a concurrent reaction which led to minor formation (<4%) of cis-[OsCl2(Him)(4)]Cl (8). The complexes were characterized by elemental analysis, IR spectroscopy, UV-vis spectroscopy, ESI mass spectrometry, cyclic voltammetry, and X-ray crystallography. 5a, 7a, and 9a were found to possess remarkable antiproliferative activity in vitro against A549 (non-small cell lung carcinoma), CH1 (ovarian carcinoma), and SW480 (colon carcinoma) cells, which was compared with that of related ruthenium compounds trans-[RuCl2(Hazole)(4)]Cl, where Hazole Hpz (5b), Hind (6b), Him (7b), and Hbzim (9b).