As evidenced by UV-Vis and EPR spectroscopies, the reaction of H2IrCl6·6H2O or Na2[IrCl6]·nH2O with DMSO results in a slow reduction of Ir(IV) avoiding the formation of Ir(IV) dimethyl sulfoxide complexes in measurable quantities. More specifically, we successfully isolated and solved the crystal structure of a sodium hexachloridoiridate(III), Na3[IrCl6]·2H2O, as a product of Na2[IrCl6]·nH2O reduction in an acetone solution. Furthermore, it was shown that [IrCl5(Me2CO)]- species is gradually formed in an acetone solution of H2IrCl6·6H2O upon storage. The reaction of DMSO with aged acetone solution of H2IrCl6·6H2O dominated by [IrCl5(Me2CO)]- affords a novel iridium(IV) chloride-dimethyl sulfoxide salt [H(dmso)2][IrCl5(dmso-κO)] (1). The compound was characterized by various spectroscopies (IR, EPR, UV-Vis) and X-ray diffraction techniques applied both to single-crystal and polycrystalline powder. The DMSO ligand is coordinated to the iridium site via the oxygen atom. New polymorph modifications of known iridium(III) complexes [H(dmso)2][trans-IrCl4(dmso-κS)2] and [H(dmso)][trans-IrCl4(dmso-κS)2] were isolated and structurally elucidated as byproducts of the above reaction.