The rates of hydrogen ion exchange on phosphonic acid ligands and that of phosphonate ligand exchange on selected uranyl-phosphonate complexes have been investigated by dynamic NMR spectroscopy. The spin-spin relaxation time (In(1/T-2)) for H+ exchange on the free ligands exhibits a parabolic dependence on reciprocal temperature (1/T (K-1)). The empirical fit parameters are correlated with the activation parameters (Delta H*, Delta S*, Delta C-p*) by adapting the statistical mechanical framework developed by Braibanti ct al. to the Eyring activated complex theory. The correlation of the apparent activation enthalpy with temperature indicates that 6-11 water molecules participate in the transition from reactants to activated complex for ligand proton exchange reactions: depending on the ligand. Mechanistic details of ligand exchange reactions of phosphonic acid complexants onto the corresponding uranium(VI) complexes are not fully developed, but the negative values for Delta S* imply increased order in the transition to the activated complex. Though Delta H* for ligand exchange covers a range of 15 kJ/mol, the free energy of activation (Delta G*) is nearly constant for the series of ligands, implying substantial entropy compensation in the activation process. Application of the Marcus relationship defines a correlation between Delta G* and the thermodynamic stability (Delta G degrees) for the 1:2 uranyl complexes with methanediphosphonic acid and 1-hydroxyelhane-1,1-diphosphonic acid.