In aqueous solutions under mild conditions, [Ru(H2O)(6)](2+) was reacted with various water-soluble tertiary phosphines. As determined by multinuclear NMR spectroscopy, reactions with the sulfonated arylphosphines L = mtppms, ptppms and mtppts yielded only the mono- and bisphosphine complexes, [Ru(H2O)(5)L](2+), cis-[Ru(H2O)(4)L-2](2+), and trans-[Ru(H2O)(4)L-2](2+) even in a high ligand excess. With the small aliphatic phosphine L = 1,3,5-triaza-7-phosphatricyclo-[,7)]decane (pta) at [L] : [Ru] = 12 : 1, the tris- and tetrakisphosphino species, [Ru(H2O)(3)(pta)(3)](2+), [Ru(H2O)(2)(pta)(4)](2+), [Ru(H2O)(OH)(pta)(4)](+), and [Ru(OH)(2)(pta)(4)] were also detected, albeit in minor quantities. These results have significance for the in situ preparation of Ru(II)-tertiary phosphine catalysts. The structures of the complexes trans-[Ru(H2O)(4)(ptaMe)(2)](tos)(4).2H(2)O, trans-[Ru(H2O)(4)(ptaH)(2)](tos)(4).2H(2)O, and trans-mer-[RuI2(H2O)-(ptaMe)(3)]I-3.2H(2)O, containing protonated or methylated pta ligands (ptaH and ptaMe, respectively) were determined by single crystal X-ray diffraction.