Three ruthenium(III) complexes containing 1H-1,2,4-triazole (Htrz), viz., (H(2)trz)[cis-RuCl4(Htrz)(2)], 1, (H(2)trz)[trans-RuCl4(Htrz)(2)], 2, and (Ph3PCH2Ph)[trans-RuCl4(Htrz)(2)], 3, have been synthesized by reaction between RuCl3 and excess of the triazole in 2.38 M HCl (1 and 2), while 3 was obtained by metathesis of 2 and [Ph3PCH2Ph]Cl in water. The products were characterized by IR, UV-vis, electrospray mass spectrometry, cyclic voltammetry, and X-ray crystallography (1 and 3). X-ray diffraction study revealed cis and trans arrangements of the triazole ligands in 1 and 3, correspondingly, and unprecedented monodentate coordination of the triazole through N2 and stabilization of its 4H tautomeric form, which is the disfavored one for the free triazole. The cytotoxicity of 1 and 2 has been assayed in three human carcinoma cell lines SW480, HT29 (colon carcinoma), and SK-BR-3 (mammary carcinoma). Both compounds exhibit antiproliferative activity in vitro. Time-dependent response of all three lines to 1 and 2 and a structure-activity relationship, i.e., higher activity of the trans-isomer 2 than that of cis-species 1, have been observed.