Four novel cis-ruthenium bis(bipyridine) sulfoxide complexes with the general formula cis-[Ru(A)₂(B)(Cl)]X (2, A = 2,2′-bipyridine (bpy), B = dimethyl sulfoxide (DMSO); 3, A = 4,4′-dimethyl-2,2′-bipyridine (dmbpy), B = DMSO; 4, A = bpy, B = tetramethylene sulfoxide (TMSO); and 5, A = dmbpy, B = TMSO; X = Cl^–, I^–, PF₆^– or ClO₄^–) were synthesized from cis-[Ru(bpy)₂Cl₂] 1 or trans-[Ru(dmbpy)₂Cl₂] 6 in the substrate sulfoxide solutions at 60–120 °C, i.e. by a thermal process. This cis selectivity is in contrast to previously reported results. However, photoirradiation of 1 in the presence of DMSO selectively produced trans-ruthenium bis(bipyridine) sulfoxide; when 6 was photoirradiated in the presence of DMSO cis-[Ru(dmbpy)₂(DMSO)Cl]Cl was the sole product. These complexes were fully characterized by elemental analysis, IR, UV/vis, ¹H, ¹³C and 2-D NMR spectroscopy. The sulfoxide ligands co-ordinate through a Ru–S bond in all cases. The NMR studies of 2 imply that no rotation around the Ru–S bond occurs, in accord with quantum mechanics calculations. Crystallographic structural determinations of 2·PF₆^– (from acetone–diethyl ether) and 2·I^– (from water) showed that both complexes share similar octahedral geometries, but different conformations were found for the sulfoxide ligands with Cl–Ru–S–O dihedral angles of 121.6 and 56.3°, respectively, thus demonstrating that a different energetically favored conformation may exist in low or high dielectric environment. The stability of complexes 2–5 allowed separation into their Δ and Λ enantiomers, and the circular dichroism spectra were obtained. Thermal substitution reactions were also carried out using 2·Cl^– which was converted into cis-[Ru(bpy)₂Cl₂] 1 or cis-[Ru(bpy)₂I₂] 7. Several examples in which the resolved complex Δ-2·PF₆^– reacts with bipyridine nucleophiles with nearly complete retention of chirality are also given.