Hydrolysis of an organometallic cation, [Ru(η(6)-p-cym)(H(2)O)(3)](2+) (p-cym = 1-isopropyl-4-methylbenzene), in the presence of 0.20 M KNO(3) or KCl as supporting electrolyte was studied in detail with the combined use of pH-potentiometry, (1)H-NMR, UV-VIS and ESI-TOF-MS. Stoichiometry and stability constants of chlorido, hydroxido and mixed chlorido-hydroxido complexes formed in aqueous solution have been determined. At pH < 4.0 where hydrolysis of [Ru(η(6)-p-cym)(H(2)O)(3)](2+) is negligible with increasing chloride ion concentration two chlorido complexes, [Ru(η(6)-p-cym)(H(2)O)(2)Cl](+) and [{Ru(η(6)-p-cym)}(2)(μ(2)-Cl)(3)](+), are detectable. At pH > 5.0, in chloride ion free samples the exclusive formation of [{Ru(η(6)-p-cym)}(2)(μ(2)-OH)(3)](+) is found. However, if chloride ion is present (in the range 0-3.50 M) novel mixed chlorido-hydroxido species, [{Ru(η(6)-p-cym)}(2)(μ(2)-OH)(2)(μ(2)-Cl)](+) and [{Ru(η(6)-p-cym)}(2)(μ(2)-OH)(μ(2)-Cl)(2)](+) can also be identified at pH > 4.0. The results obtained in this study may help in rationalizing the solution behaviour of half-sandwich [Ru(η(6)-p-cym)(XY)Z] type complexes which, after dissociation of both the monodentate Z and the chelating XY, are capable of yielding the free aqua species [Ru(η(6)-p-cym)(H(2)O)(3)](2+). Our results demonstrate that different chloride ion concentrations can influence the speciation in the acidic pH range but at biologically relevant conditions (pH = 7.4, c(Cl(-)) = 0.16 M) and at c(M) = 1 μM [{Ru(η(6)-p-cym)}(2)(μ(2)-OH)(3)](+) is predominant in the absence of any coordinating ligands.