The rate constants and the activation parameters for the exchange between water solvent and [U(H2O)(10)](4+) and [UF(H2O)(9)](3+), and a lower limit for the rate constant at room temperature for [Th(H2O)(10)](4+), were determined by O-17 NMR spectroscopy in the temperature range 255-305 K. The experiments were made at different constant hydrogen ion concentrations, which varied between 0.16 and 0.8 mol kg(-1). The Th(IV) system was investigated using Tb3+ as a shift reagent. The following kinetic parameters at 25 degrees C were obtained: k(ex) = (5.4 +/- 0.6) 10(6) s (-1), Delta H-not equal:= 34 +/- 3 kJ mol(-1), Delta S-not equal = -16 +/- 10 J mol(-1) K-1 for U4+(aq), k(ex) = (5.5 +/- 0.7) 10(6) s (-1), Delta H-not equal = 36 +/- 4 kJ mol(-1), Delta S-not equal = 3 +/- 15 J mol(-1) K-1 for UF3+(aq), and k(ex) > 5 10(7) s (-1) for Th4+(aq), where the uncertainty is given at the 2 sigma-level. This is the first experimental information on the kinetic parameters for the exchange of water for any M4+ ion. There is no information on the rates and mechanisms of ligand substitutions involving other mono-dentate ligands, hence the mechanistic interpretation of the data is by necessity provisional. The kinetic data and the known ground-state geometry with a coordination number of 10 +/- 1 for the Th(IV) and U(IV) complexes suggest a dissociatively activated interchange mechanism. There is no noticeable effect of coordination of one fluoride or one hydroxide to U(IV) on the water exchange rate. This is unusual, for other metal ions there is a strong labilizing of coordinated water when a second ligand is bonded, e.g., in complexes of aluminum and some d-transition elements. In previous studies of the rates and mechanisms of ligand exchange in uranium(VI) systems we found a strong decrease in the lability of coordinated water in some fluoride containing complexes.