In this paper we report the synthesis and characterization of the following ruthenium(II) and ruthenium(III) complexes with tetramethylene sulfoxide (TMSO): cis-RuCl2(TMSO)4 (1), trans- RuCl2(TMSO)4 (2), the corresponding dibromo derivatives cis- and trans-RuBr2(TMSO)4 (3 and 4, respectively), (TMSO)H[trans-Ru(TMSO)2Cl4] (5) and mer-RuCl3(TMSO)3 (6). Most of the reported complexes are described here for the first time, together with a new, easy synthetic path for the previously known complexes 1 and 3. The chemical behavior of 1 and 2 in solution of aprotic, non- coordinating solvents is described. Among the Ru(II) derivatives, the cis isomers are thermodynamically more stable and a photochemically driven cis to trans isomerization reaction is observed in tetramethylene sulfoxide solution. We also report the crystal structures of cis-RuCl2(TMSO)4 (1) and (TMSO)H[trans- Ru(TMSO)2Cl4] (5), as determined by three dimensional X-ray analyses. Crystal data: 1, a=9.104(2), b=11.317(2), c=21.867(6) A, β=90.62(2)°, monoclinic, space group P21/c, Z=4; 5, a=14.642(4), b=14.999(4), c=9.667(4) A, β=103.57(1)°, monoclinic, space group P1/c, Z=4. Least-squares refinement based on 3813 (1) and 4706 (5) reflections converged to R=0.032 and 0.033 for 1 and 5, respectively. In 1, all the TMSO ligands are S-bonded to Ru, with average RuS bond distances of 2.355(6) (trans to S) and 2.273(1) (trans to Cl) A. The TMSO ligands in 5 are both S-bonded to Ru (av. 2.33(1) A). The protonated TMSO molecule is hydrogen bonded to one of the two crystallographically independent anions.