Complexation of U-VI with 1-hydroxyethane-1,1-diphosphonic acid (HEDPA) in acidic to basic solutions has been studied with multiple techniques. A number of 1:1 (UO2H3L), 1:2 (UO2HjL2 where j = +4, +3, +2, +1, 0, and -1), and 2:2 [(UO2)(2)HjL2 where j = +1, 0, and -1] complexes form, but the 1:2 complexes are the major species in a wide pH range. Thermodynamic parameters (formation constants and enthalpy and entropy of complexation) were determined by potentiometry and calorimetry. Data indicate that the complexation of U-VI with HEDPA is exothermic, favored by the enthalpy of complexation. This is in contrast to the complexation of U-VI with dicarboxylic acids in which the enthalpy term usually is unfavorable. Results from electrospray ionization mass spectrometry and P-31 NMR have confirmed the presence of 1:1, 1:2, and 2:2 U(VI)HEDPA complexes.