Lanthanide hexafluoroacetylacetonate ternary complexes with 2,2'-bipyridine-N,N'-dioxide, [Ln(hfa)3(bpyO2)], were synthesized for Ln = Eu, Gd, Tb, and Lu and fully characterized by elemental, thermal, and mass-spectrometric analyses. The X-ray crystal structure of [Eu(hfa) 3(bpyO2)].0.5C6H6 reveals an octa-coordinate metal ion lying in a severely distorted trigonal dodecahedron geometry; the Eu-O distances lie in the range 2.36-2.44 Å with no significant difference between hfa- and bpyO2. A detailed comparative photophysical investigation has been carried out to determine the exact influence of the introduction of bpyO2 in the inner coordination sphere of the metal ion in replacement of the two water molecules in [Ln(hfa)3(H2O)2]. While this replacement is detrimental for Tb, it leads to a 15-fold increase in the overall quantum yield for Eu. This large improvement originates from (i) a better sensitization efficiency, the ancillary ligand being responsible for 3/4 of the energy transfer, (ii) elimination of nonradiative deactivation pathways through harmonics of O-H vibrations, and (iii) reduction in the radiative lifetime. The latter influence is rarely documented, but it accounts here for ã25% increase in the intrinsic quantum yield, so that more attention should be given to this parameter when designing highly luminescent lanthanide complexes. © 2011 American Chemical Society.