Data from potentiometric titrations of Eu(CF(3)SO(3))(3) by Me(4)NOH in 80% v/v DMSO/water agree equally well with formation of two sets of hydroxo complexes: either Eu(2)OH(5+) and Eu(OH)(n)(3-n) (n = 1-3) or Eu(2)OH(5+), Eu(2)(OH)(2)(4+), Eu(2)(OH)(5)(+) and Eu(OH)(3). Studies using direct excitation luminescence spectroscopy and time-resolved decay measurements of Eu(III) solutions containing increasing amounts of hydroxide as well as luminescence resonance energy transfer studies between Tb(III) and Eu(III) in the presence of added hydroxide support the second model with predominantly binuclear hydroxo complexes. The luminescence lifetimes of hydroxo complexes are much shorter than those of Eu(III) complexes solvated by DMSO-water mixtures and even shorter than for Eu(III) aqua complexes, but are larger than those reported for hydroxo complexes in water. This indicates possible preferential solvation by DMSO of both free Eu(III) cation and its hydroxo complexes. Kinetic studies of the cleavage of two model phosphodiesters by Eu(III) triflate in the same medium shows that reactive species are higher order hydroxo complexes Eu(2)(OH)(5)(+) and Eu(OH)(3). Reactions with Eu(OH)(3) are second or third order in the complex indicating involvement of bi or trinuclear species as the reactive forms.