In aqueous solutions under mild conditions, Ru(H2O)(6) 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, cis-Ru(H2O)(4)L-2, and trans-Ru(H2O)(4)L-2 even in a high ligand excess. With the small aliphatic phosphine L = 1,3,5-triaza-7-phosphatricyclo-[3.3.1.1(3,7)]decane (pta) at [L] : [Ru] = 12 : 1, the tris- and tetrakisphosphino species, Ru(H2O)(3)(pta)(3), Ru(H2O)(2)(pta)(4), 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)(4).2H(2)O, trans-Ru(H2O)(4)(ptaH)(2)(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.