Potentiometric and microcalorimetric titration techniques were applied for the determination of the Gibbs free energies and enthalpies of the protonation and U(VI) complexation of some carboxylic acids (formic, acetic, glycolic, and propionic acids) in 1.0 M NaClO4 solution at 25 degrees C. By using the values of Delta G determined by potentiometric titrations, the results of calorimetric titrations were analyzed to give the values of Delta H and Delta S. These enthalpy values indicated that the protonation and uranyl(VI) complexation of these carboxylates were mainly entropy-driven, that is, vertical bar-T Delta S vertical bar >> vertical bar Delta H vertical bar in Delta G = Delta H - T Delta S. The comparison of T Delta S-m values for uranyl acetate and glycolate complexation with those for europium(III) complexation revealed that the complexation of U(VI) was accompanied by larger entropy changes due to the limited space in its coordination sphere caused by the steric hindrance of two oxygens in the linear dioxo structure of uranyl ion. (c) 2007 Elsevier Ltd. All rights reserved.