Time-resolved fluorescence spectroscopy and Fourier transform IR spectroscopy have been applied to characterize the coordination environment of lipophilic complexes of Eu(3+) with bis(2-ethylhexyl)phosphoric acid (HDEHP) and (2-ethylhexyl)phosphonic acid mono(2-ethylhexyl) ester (HEH[EHP]) in 1,4-diisopropylbenzene (DIPB). The primary focus is on understanding the role of lactate (HL) in lanthanide partitioning into DIPB solutions of HDEHP or HEH[EHP] as it is employed in the TALSPEAK solvent extraction process for lanthanide separations from trivalent actinides. The broader purpose of this study is to characterize the changes that can occur in the coordination environment of lanthanide ions as metal-ion concentrations increase in nonpolar media. The optical spectroscopy studies reported here complement an earlier investigation of similar solutions using NMR spectroscopy and electrospray ionization mass spectrometry. Emission spectra of Eu(3+) complexes with HDEHP/HEH[EHP] demonstrate that, as long as the Eu(3+) concentration is maintained well below saturation of the organic extractant solution, the Eu(3+) coordination environment remains constant as both [HL](org) and [H(2)O](org) are increased. If the total organic-phase lanthanide concentration is increased (by extraction of moderate amounts of La(3+)), the (5)D(0) → (7)F(1) transition singlet splits into a doublet with a notable increase in the intensity of both (5)D(0) → (7)F(1) and (5)D(0) → (7)F(2) electronic transitions. The increased multiplicity in the emission spectra indicates that Eu(3+) ions are present in multiple coordination environments. The increased emission intensity of the 614 nm band implies an overall reduction in symmetry of the extracted Eu(3+) complex in the presence of macroscopic La(3+). Although [H(2)O](org) increases to above 1 M at high [HL](tot), this water is not associated with the Eu(3+) metal center. IR spectroscopy results confirm a direct Ln(3+)-lactate interaction at high concentrations of lanthanide and lactate in the extractant phase. At low organic-phase lanthanide concentrations, the predominant complex is almost certainly the well-known Ln(DEHP·HDEHP)(3). As lanthanide concentrations in the organic phase increase, mixed-ligand complexes with the general stoichiometry Ln(L)(n)(DEHP)(3-n) or Ln(L)(n)(DEHP·HDEHP)(3-n) become the dominant species.