The complexation of neodymium(III) and europium(III) with lactate was studied at variable temperatures by potentiometry, absorption spectrophotometry, luminescence spectroscopy, and microcalorimetry. The stability constants of three successive lactate complexes (ML2+, ML2+, and ML3(aq), where M stands for Nd and Eu and L stands for lactate) at 10, 25, 40, 55, and 70 degrees C were determined. The enthalpies of complexation at 25 degrees C were determined by microcalorimetry. Thermodynamic data show that the complexation of trivalent lanthanides (Nd3+ and Eu3+) with lactate is exothermic and the complexation becomes weaker at higher temperatures. Results from optical absorption and luminescence spectroscopy suggest that the complexes are inner-sphere chelate complexes in which the protonated a-hydroxyl group of lactate participates in the complexation.