Two diruthenium(II) complexes [RuCl 2 (p-cymene)] 2 (μ-BESE) (1), [RuCl 2 (p-cymene)] 2 (μ-BESP) (2), and the mononuclear salt [RuCl(p-cymene)(BESE)]PF 6 (3), containing the disulfoxides BESE and BESP, were synthesized and characterized by elemental analysis, and NMR and IR spectroscopies, and were shown to contain S-bound sulfoxide groups; the disulfoxides are EtS(O)(CH 2 ) n S(O)Et, where n=2 (BESE) or 3 (BESP). Complexes 1 and 3 were also characterized by X-ray crystallography. Preliminary in vitro tests of 1 and 3 were conducted using the MTT assay, which measures mitochondrial dehydrogenase activity as an indication of cell viability; these complexes showed in vitro anti-cancer activity against a human mammary cancer cell line (MDA-MB-435s) with IC 5 0 values of 360 and 55 μM, respectively.